Originally Posted by Jiminycrickets:
Depends a little on the three year old, but a few ideas: Have a "tea party." Make a tent with sheets over the dining room/kitchen table. Build a box fort. Fly paper airplanes. Go for a nature walk and pick up pinecones and pretty rocks and make a display in a shoe box. Make paper dolls. Or simple dolls with a piece of cloth and some string tied around it for arms or legs. Dance party. Scavenger hunt with pictures instead of words for the list. Dress up in any "fancy" clothes you have hanging around. Play cooking with real pots and pans. Make flour play dough. Have her draw a picture and then dictate a story about it. And my favorite: take a nap. 
These are good reminders of simple yet timeless activities for children - good contrast to the "more toys" is "better" mentality that pervades our culture.
The children will enjoy doing any of these things if you are enjoying it too!