I've used Aroma1997's Dimensional World, Twilight Forest, and Galacticraft a little bit in 1.7.10 (including visiting the added dimensions, though I haven't tried their 1.12 updates), plus Mystcraft (but that one hasn't updated to 1.12 yet, only 1.11). There's also the Deep Dark from Extra Utilities 2, which is a mod I've used, but I haven't visited the dimension.
I just downloaded Bukkit! I'm looking for a mod that'll add some more life to my minecraft server. New structures, landscapes, or mobs, npcs and pets.
I'm only playing with my girlfriend. I'm not hosting a huge server.
You haven't specified what Minecraft version you'll be playing, but if it's 1.12.2, the betas for Millenaire might interest you.
I'm running the latest version of Minecraft. Isn't there one where you can travel to another dimension (similar to the nether)
There are a lot of mods that add dimensions. I'm not especially familiar with them, so I'll link you the category:
https://minecraft.curseforge.com/mc-mods/world-gen/world-dimensions?filter-game-version=2020709689:6756&filter-sort=4
I've used Aroma1997's Dimensional World, Twilight Forest, and Galacticraft a little bit in 1.7.10 (including visiting the added dimensions, though I haven't tried their 1.12 updates), plus Mystcraft (but that one hasn't updated to 1.12 yet, only 1.11). There's also the Deep Dark from Extra Utilities 2, which is a mod I've used, but I haven't visited the dimension.