It happens to all of us. We play on the same world for weeks, and eventually run out of things to build or motivation to play. Mutliplayer is no doubt a great alternative, but what can you do if your sever's down? Or you internet goes out? HOW WILL YOU COPE WITHOUT YOUR MINECRAFT FIX?!
We'll, I'd like to recommend this mod: Mo' Creatures, created by DrZhark on the MineCraft Forums. This Mod adds a whole bunch of new creatures to the game, including rabbits, birds, werewolves, sharks, and much more. There is even an entire horse taming/riding/breeding system which finally gives use to those otherwise useless saddles. Give it a shot, you won't be disappointed.
Some pics of Mo' Creatures: