What I do is I try to find an obscure corner in a server, and get wrenches + diamond pickaxe as quickly as possible. Then, I go into the nether, and use that as a quick express way to other people's bases (because they ALWAYS put the nether portal next to their base). Then, at night when no one is around, I emerge and just start messin' with stuff. First, I like to use the wrench on all of their machines and make it so random pipes aren't connected, and the machines will be facing the wrong way for some reason. Then, I like to go into people's chests, but I don't take anything. I just re-order all of their stuff, stashing random things into random chests. Then, I jump on at least one block of farm land, and cause it to revert back to normal dirt. Then, I'll remove a few torches in the corner of the house, so when they log in, monsters will keep coming out of nowhere for some reason. I also like to grab tools and use up a bunch of their durability by placing and breaking the same blocks over again, but I always put the tools back.
My greatest savagery comes from when I destroy someone's bed, then put it back down again. Then, when they die, they'll respawn in the middle of nowhere. It is only then that they'll know the true fury of the server gremlin.
j/k. On a serious note, what I do usually isn't anything special. I make a hole in the ground (usually where I spawned), try to procure some food and stuffs, explore a few caverns to build up enough equipment, then I'll go off to try and make a build somewhere. It could be a gigantic pirate ship. It could be a hanging castle. It could be a pyramid with a stupid looking dragon/alligator head for an entrance. Who knows what will take my fancy when I play next time.
What I like to do is pick a mod that I'll specialize in, and try to use that mod as much as possible. It usually ends up being thaumcraft. Until I get to that point, I play the game mostly like vanilla minecraft. I'll try to get some very basic machines up to double ore production, and usually that is a TiC smeltery.
So my advice would be to pick out a mod, and roll with it. The more obscure and unknown, the better. Try to use mods that you aren't familiar with, that way each play-through seems like something new.