What I've spend too much money on: Tech gadgets, DSLRs, LEGO sets, neat things, books I don't read, stuff to put on shelves. Especially if you have too much money. At first you don't think a lot if you need things <$10, when your income grows so does your "don't-think-spending-limit" to $100, then $1000.
Not stupid: Books you really read, long term investing, making experiences.
Today I have a monthly "Make-your-live-lighter-day" where I give away/throw away things.