Sometimes with little ones you need some boredom busters. Here are a few ideas, not much money. Maybe a little messy, but that is part of the fun.
Ghost zip line from My Nearest and Dearest. I could see this becoming a fun experiment to see just what will go the fastest down the zip line.
Kids Activities Blog has a cute art project using toilet paper rolls.
Oh boy learning and fun! B-Inspired Mama has a fun why to learn numbers with toy cars.
Babble has 25 rainy day activities for kids. I like the this tic tac toe game as it involves movement.
Here is one for older kids. Make your own chess set from nuts and bolts. The one is from Cub Scouts Ideas.
Frugal Fun for Boys has a fun design for Legos.