Magna-Tiles
• $65
• Grasshopper
If I didn't know for a fact that my son played with these every day at preschool, I might rule them out for cost, but pro-rated, these are a wise investment. Each piece has magnetic edges for open-ended construction.
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They were a good investment for us. My son started playing with them at pre-school, and loved them so much that I bought some for home. He was three when we got them, and is just about to turn 7 and still uses them to built mazes and bases for his superheroes. They are easy to clean, and they pack up very neatly and compactly.
My son got the cousin of magna tiles as a gift. I found that they are less expensive but just as fun--still definitely get the creative/building juices flowing.