Oh those Euro coins from Greece reminded me of my semester in Thessaloniki! Sigh! I'd create a composition with several bills and then, with a nice background (like pretty wrapping paper or a wallpaper scrap) fit it in a larger and easier to find frame.

I'm not sure how much money you wanted to spend, but you can always have a the bills custom framed, eliminating the size problem

the question poses an even more interesting question about the economy.....

It'd be great if you could find a frame that sandwiches the framed material between two sheets of glass. The thing about coins and bills is that both sides are usually very interesting, so it'd be good to be able to pull the frame off the wall and see both sides. This would be easy with bills, but tough with coins since they are all different thicknesses.

I'm thinking Clips frames from IKEA (they're like Dharmabum described) for the bills.

I agree with dharmabum and Enamorada: suspend them in floating frames and instead of hanging them, put the bills in a display case so your visitors can look at them without the possibility of damage.

Might try looking at Numismatist supply catalogs for ideas - The thing you want to be careful about with any paper and metal product is oxidizing, or tarnishing, if you care about having them shiny, or the paper degrading (as it is wont to do). You might find the perfect thing in there, or even just phone 'em up and ask. Most guys are pretty nice, and they'll let you know how to keep the stuff from breaking down over time.

Or rather than keeping them on display, you could put them to good use.
Because banks will only exchange paper money, on many international flights the flight attendants pass out envelopes from Unicef to collect unwanted loose change. The collections support charitable efforts around the world. Not sure if this will work, however, with your older coins from countries that have since converted to the Euro.

Martha Stewart Living had a great way to display currency and other souvenirs a few years ago. Check it out here:

http://www.marthastewart.com/article/do-it-yourself-art-travel-collages?autonomy_kw=travel%20collage&rsc=header_1

I hot-glued magnets to my foreign coins and use them on my metal bulletin board.

Depending on how many you have, I saw something cool at a bar in Austin. The back-splash (behind the sink) was basically tiled with pennies. You could do the same thing with your foreign coins (and maybe supplement with some pennies you've collected). The pennies were, I'm guessing, epoxied over. May or may-not be a do-it-yourself project depending on your level of expertise. The overall effect was very eyecatching, like shiny tiles of round copper.

If you wanted to do something less permanent, you could "tile" your coins and bills on a small canvas frame and then epoxy over that.

Both of these suggestions would render your collection value-less so hopefully you don't have anything thats too special.

The epoxy I'm referring to is available at Home Depot or most hardware stores. Its used often by artists over acrylic canvases. You apply, smooth it out, and dry it with a hairdryer.

Here are some ideas, if you don't mind destroying the money (I've done all of these things and was quite satisfied with the results): put sticky tape on the back of coins and arrange directly on the wall in a pattern; use the bills to create decoupaged bowls or cover small boxes with them (instead of gift wrapping); self-laminate the bills and use them as bookmarks (or give as gifts with travel books); glue magnets on the back of coins and use on the fridge.

I like the magnet idea for the coins. Simple, but doesn't have to look tacky if not on the fridge. As for the bills, I would do something like this (a whole wall):
http://urbangraceinteriors.typepad.com/the_blog/2008/08/curdled-milk.html
http://urbangraceinteriors.typepad.com/the_blog/2008/05/did-i-tell-you.html

I like the idea of floating frames...

In my son's room I just put up a grouping of shadow box frames with various things in them. One of the smaller ones has a pound in it. I put it in with a double sided foam sticker from the scrapbook section of the craft store. I used some neat papers as the background. I really like the idea of a composition of money.

I frame bills already in 5'X7' frames with a 4X6 matte. I prefer having a larger frame with a single bill in it to really give each bill their deserved attention. It sucks to have to do it, but I choose a favorite side and deal with the fact that the other isn't showcased.

If that's too mych for you, maybe you can make a mobile with coins and bills (you could laminate the bills so it looks neater, if you like) The mobile can be in the corner of a room, doesn't have to be over a crib, or even arranged around a suspended bare bulb, to act as a light fixture? (insiration below)

http://www.etsy.com/view_listing.php?ref=sr_gallery_17&listing_id=6820689

mych = much

Cheaper than custom frames are a variety of sized frames in a matching style/color, and get custom matting to tie them together. Don't be afraid of matting the bills in different parts of the frame...it doesn't always have to be centered. It looks more interesting if you don't.

Another idea for putting the coins in a picture frame:
If you add a sheet of magnet in front of the backing board, the coins stick to the magnet and you can re-arrange or remove them whenever you want.