For a 5yo?
Pattern blocks. No question.
They cost in the $15-20 range for a big bucket of them. They're sold on educational websites (and stores, presumably) like
www.didax.com. They're like a big ol' bin of tangram shapes that can be used for geometry, fractions, multiplication and division practice, art, imaginative play, etc.
My kids were OBSESSED with them for a year or two around age 5-ish. (Older for DS8, younger for DS5.) We did math with them, but they loved them best for making Transformers out of them. They still get them out occasionally to make Batman or Star Wars characters out of them. Best money I ever spent!
