My friends and I have a minecraft server that we have had for quite some time. We have built castles and boats and dug to the depths of the ocean and had wars that involved a lot of TNT and server crashing. Frankly we are bored. I have seen some mods that change minecraft from a "creative sandbox" game with no goal but the one you give it into a "creatively survive with nothing and here is your goal" kind of game.
We are looking for a mod that gives us a goal that we can accomplish. Specifically I would like this goal to be survival against all odds rather than the current goal of "be creative while surviving easily."
My question how do we go about acquiring and playing these sort of mods.
Edit: tried to clarify question.