Adventura

There are DOZENS of mods specifically for exploration and adventuring. Endless styles of dungeons, and massive cave systems with their own biomes, oceans, etc.
But at night time, there are hundreds of new mobs, and modified mobs such as those that can break blocks to get to you, destroy your torches, and a GRUE, which will insta-kill you if you're in the dark without a light!!
There's also RPG elements and NPC's, RPG Villages, and leveling system.
And don't worry about the mobs, there are tons of weapon/armor upgrades you can get to take care of them.

In a nutshell; explore during the day, and pray you survive the nights.