Nanoblocks - LOVE
Anyone who likes Legos will likely enjoy Nanoblocks. They are like teeny-tiny Legoes. Most of the sets run around $8 for animals and $15 for landmark buildings. They have a couple of deluxe sets that are essentially scale models of landmarks and run $100+. Some examples:
www.amazon.com/Nanoblock-Castle-Neuschwanstein-550-pcs/dp/B004JXWTJE/ref=sr_1_2?s=toys-and-games&ie=UTF8&qid=1314111357&sr=1-2" target="_blank">Typical
DeluxeDK bought me the typical castle model for my birthday and I had a lot of fun putting it together. It probably took about 2-3 hours. The directions are similar to Lego, and very easy to follow. They also give you a few extra pieces in case you lose one.
I bought the Eiffel Tower and Taj Mahal but still need to assemble. I'd say these are moreso for teens- adults because little kids would have way too much trouble with the tiny pieces.