TITLE: What are some mathematically productive ways to waste time?
QUESTION [21 upvotes]: What are some productive things that can be done (other than directly studying Mathematics) during leisure time that has a side effect to improve oneself at Mathematics? For example, reading Mathematics Stack Exchange.
Inspired by What to do while compiling?

REPLY [3 votes]: Try to do putnam competition tests. You don't really learn a lot of theory but it's a good exercise in coming up with proofs.