The wild fairy houses are pretty cool, and something simple to do when out walking in the woods. Fairy houses for your garden or yard can be much more elaborate.
Here is a variety of whimsical and wonderful fairy houses collected from a variety of sources. I'm thinking this would be a great project for Beltane, don't you?
I also found this Fairy House Tour of Homes slide show with some really wonderful little houses. This is one of them:
Here's an article on "how-to make a fairy house" and another cool one on "how to make a doll house into a fairy house". So, if you're not sure how to begin, you might want to take a look at that. After the construction is complete, you can check daily to see if fairies have been in the house. This will be apparent when objects had been moved or added to the décor.