Yeah not sure if cruising around counts as a side activity, but that would have to be my all time favorite thing to do. Sometimes it pays off literally because I end up running into side missions to do.
My favorite side mission is the photography. I haven't got the game but I see alot of people do it. The photographer is really funny because he acts like even if he gets killed by a celeb he will still get paid. It's also funny to see how much time RockStar has put into what clebs do.
Idk why, but playing tennis on this game is really fun for me..