For parties, people will always gather near the food, and stay on one deck mostly, it just happens that way. But everyday use to make it look more together, if you are a limited budget, getting cushions and outdoor pillows that all work. Using the same type of flowers in similar planters can bring it together. I can't tell what kind of furniture you have but painting it all the same or similar colors. Then I would think about what you use that space for, maybe one dining, one reading with coffee, one laying out in the sun? Just making different zones that are easy to read can help to make the space not look like a big cafe. And have fun, with outdoor spaces you can have a lot more whimsy in the space.

Like @eamoxie said, people gather near the food. If you don't want people to crowd only on one deck, spread the food out. :)

Are you keeping all your furniture? If so, I'd try to foster a little unification by spray painting the metal parts all the same fun color like turquoise and then putting some inexpensive exterior cushions on the chairs in a contrasting color. I'd also artfully string lights with bulbs from the top railing to the fence and then below the deck (are some already there?) and on the top level.

If money were no object? Pressure wash and restain the whole shebang, collect some awesome outdoor furniture with gorgeous sunbrella fabric and rig a pulley system from the bottom to top level so you can easily sit atop your party palace and eat like an outdoor king.

Looks like a wonderful place to spend the coming summer evenings! Good Luck :)

oops, I mean queen :)

It looks like you have a sweet little area under the deck with curtains and small globe string lights. Is that off your bedroom or the kitchen? There seem to be a lot of different seating areas and patio sets. If you remove the hot tub you could do built in bench seating in an L shape along the fence and put a table in the middle or fire pit. If you are only using the mish-mash of donated items you have now it might help to spray paint them all the same color or use pillows with a similar color scheme to pull everything together. Below are a few links to some inspiration I've saved over the years for outdoor spaces. Maybe you can get some ideas from them as well.

http://pinterest.com/pin/22464449/
http://pinterest.com/pin/21905731/
http://pinterest.com/pin/17361966/

Below is one of my favorite spaces. You could do an art wall frame up a piece of plywood prime it and then do a colorful or graphic pattern/design.
http://pinterest.com/pin/15501335/

Outdoor rugs can help define areas within a bigger space.
http://pinterest.com/pin/15491594/

http://pinterest.com/pin/22037608/

This seating area is just painted pallets with cushions on top.
http://pinterest.com/pin/19168017/

http://pinterest.com/pin/19166721/
http://pinterest.com/pin/15803985/

Also if you do any planting—adding spot lights to the base of trees along the fence line adds to the ambiance in the evenings.

Wait, why are you getting rid of the hot-tub? I'm pro-hot-tub.

I'd choose a color palette and use it throughout. Buy the same type of planters and use similar colors of flowers/plants on both levels. A few potted trees would also look great!

I agree with California Child - you could probably keep the donated furniture and either give it all a fresh coat of matching paint and/or matching cushions to make it feel like it goes together. If you want to play up the party area idea, maybe find or build some counter/bar height tables along the outer wall of the house. You could even put stools underneath for additional seating. One sort of out-there thought: I wonder if you could plant a cute, dwarf tree in the hot tub space? Or a mini garden?

Others have mentioned these already, and I'm in agreement:

-Pressure wash and re-seal the deck -- the wood is looking a little worn, so this will help with its longevity in addition to freshening it up.

-Pressure wash the furniture and spray it all the same color or in a palette of matching colors. You can buy matching pillows or umbrellas to tie them all together.

-Start up some container gardening or hanging planters -- even though there's a lot of greenery around the deck, the deck itself looks a bit barren.

It looks like a great space!

1. Make the downstairs deck the public area for parties. The spiral staircase is a big detractor -- hard to get up and down when its crowded and you are drinking.
2. Therefore, set up the upper deck as living room -- a place for people to sit in small groups and chat, sunbathe, read, whatever. An outdoor living room. Get an outdoor rug to anchor the space, some outdoor pillows, and a couple of chaise lounges. Shade is very important to be comfortable, so get an umbrella and a stand, or two. I have two little ones on my deck, one for each chair. Very resorty and luxurious. Also get a bunch of lanterns -- ikea or pottery barn or pier one have good choises. Fill them with candles in glass -- so they dont melt into a puddle in the sun. Use those for lighting at night in addition to twinkle lights. There's nothing like candle light.
3. Make downstairs party space: a good BBQ with a good table for setting out food or prepping what you are cooking. Get a bunch of outdoor sling back chairs in great stripes. They fold up and out of the way. Get a couple of large trees in pots to anchor and soften corners. Grow some vines on the fence (jasmine is wonderful if your in the right zone and you get enough light, which is not that much really. Smells awesome). Get an outdoor dining table and some chairs -- maybe folding wood or metal ones. More lanterns, another rug -- maybe that matches. Tie the two spaces together with your fabrics and colors and rugs. Done!

pamh
howtorunyourlife.blogspot.com

Great deck, wow! Paint furniture to match. Get coordinating pillows (or recover ones yourself, easy to do and often the outdoor fabric is on sale this time of year) to tie the spaces together. If you want people to spread out more, make the farther away areas more inviting w/more seating and fun details. I love these funky porch curtains:
http://www.flickr.com/photos/all_about_evelyn/4760368642/in/photostream/
but something simple and white would be inviting up above if you don't want to do anything so dramatic!

Remove the upper deck and lower it so you have one level deck. Then build a set of stairs from upstairs down the side of the deck to the ground level, if you need the stairs. Now you have one big deck and the party doesn't need to be split up. Furniture arranging is easier, too, and it'll feel like lots more space. Maybe it's not a consideration for you but that's what I'd do. The two levels would make me crazy.

Thank you all for your insight and suggestions!

First, we're getting rid of the hot tub because it's broken. We've had it fixed twice since we moved in, but it's from the 80s, so I think it's past it's prime. Also, that way we can keep the blue table and move it over to where the tub is now for a more spacious eating area!

Second, we'll definately splurge on some more matching textiles. I think you're right, that would go a long way in sprucing things up! The same goes for container gardens; in fact, now I'm thinking that once we move the table to the hot tub area the lower level would be great for bench-style seating and a well-matched container garden. I'm actually a potter, so it would be an awesome project to make sets of pots specifically for this purpose!

pamh - I like the idea of an outdoor rug, and have wavered on it a few times, but I'm not sure how laying a rug over old wood would go... anyone have experience with that? Does it lead to too much rot?

CaliforniaChild - we do have several strands of bulb lights under the top deck, above our grill which are awesome. We love them :)

ps -Danica, thanks for the links!

If you want people on the upstairs deck, put the bar up there.

I would section off a part of it and use it as garden seating. Lots of potted plants, herbs, vegetables, and Adirondack chairs. A nice quiet place to relax and talk during a party. And to read in during normal hours :)

Lots of shade bc hanging out in the sun is very bad for you.