I haven't explored those much. Any suggestions?
Well, a favourite of mine is the Biometerrain mod that lets you change all kinds of settings in the terrain generator so you can have larger biomes, floating islands, more dungeons, different ore distributions and so on. The best part of this mod are the new tree types that appear in certain biomes.
There are also various mods that add mobs like humans, bears, wolves, lions and even cave-digging trolls and rideable horses/unicorns/pegasi.
Other mods try to balance things up a bit, such as giving you more stairs for your sticks, or letting you turn wool into thread.
The "Fancypack" mod adds a lot of new items like chairs, tables, clay crafts, bone crafts, lanterns, metal crafts, roads, potted plants and farmable versions of each plant.
The "Minecart Mania" mod lets you place boosters, switches, delays, reversals and more on tracks so you can make proper transportation systems.
The "Controllerblock" mod allows you to place blocks that change type or appear/disappear when the controller block is powered. Awesome for making drawbridges, hidden doors, traps and even moving windmills.
I can just keep going though, the variety of mods is near endless. My favourite is probably the one that lets you pick up chickens and float down from cliffs with them.