Mugs and plastic cups make great little caves/hideouts
You can also glue together rocks to make a little cave
ping pong balls that float make for lots of fun if bettas are large enough to move them
pretty colored paper can be used for a background as can tin foil and cut up t-shirts
large tupperware tubs make GREAT tanks!
Screaning can be bought very cheep from hardware stores and set up as a divider or lid
filters can be made from an air pump, gravel, carbon, and gaterade/ water bottle
small pots can be sanded and used as caves for bettas also.
I'll post more when I think of them, these were just a few ideas!