Poker is always fun. texas hold em FTW!! lol cheap too, can play for free or start a $5 table. that's always a good time. Pool? My friends and I used to have pool tournaments with each other, eventually we did a few actual tournaments for money, was a good time. (should do that more, need to bust out my parent's pool table!)
take up hobbyist level RC cars, they are wicked fun and waste time like no other.
model rocketry is fun.
umm... I dunno :p camping is awesome, mountain biking is fun, lots of stuff to do outdoors. Mini vacations? I love to just get in a car and listen to music and drive. with no particular destination, just hit up a highway you haven't driven on and go for a while.