Coconut shells are fun to make, and only take around 30 minutes, the most an hour. They don't cost much either. Just crack one in half, scrape out the coconut inside and sand the edges down and the fuzzies on the outside. My bettas like them and have been going in and out of them and they make your tank look tropical. :D
Other things, you could glue (aquarium sealent!) colorful glass pebbles togeter to make a cave or an ornament, like a turtle or something. I had some blue and white ones that I thought of making a dolphin out of, but I ended up using them as little gyms in Autumns aruarium.