I think working on prime numbers is a nice hobby. I haven't read a lot about them (I'm not good at math), all I know they seem to be somewhat "random", although I don't believe they are. The way composite numbers are made up appears to be two types: divisible by two or made from a number of >2 primes in a certain way (some kind of prime sequence itself) where I think there is some kind of logic in that, but I don't yet know what.
Did I start a topic about this already? Can't remember..