What Can I Have 3yr olds Do For Independent Activities?

I just started work at a day care, and within a week convinced them I could handle my own afternoon cl of fourteen three year olds. Its going great and I love it! They have activities with their morning teacher, but when I take over, its just snack time, recess, free play (read chaos) then movie time. During their free play time, I've got to vacuum their play rug, sweep and mop. So, I need them to have a table activity while I vacuum. So far, I've just had them working on puzzles for this, but I can tell this is getting old! So I need some ideas for their table activities. I only need about fifteen minutes. Ideally, it should be easy enough for them to almost if not completely do on their own with supervision, low-mess (*hopeful*), at least a bit educational and Fun! for them. I'd also like ideas for some activities for them while I have to sweep and mop. While I'm doing this, they stay on a large area rug. I need at least 20 minutes, but these activities could go up to thirty minutes with no problem (I'm all for minimizing how much time they spend eyes glued to Tom and Jerry!). Also, cheap/reusable materials for these activities as much as possible!!!!!