There are countless mods that make Skyrim even colder and harsher than it already is, but Project Rainforest, a Skyrim Legendary Edition mod, does the opposite. Snowy tundras turn to tropical grasslands, frozen lakes to warm swamps, and icicles to lily pads.
It changes the whole of Skyrim, Fort Dawnguard, Falmer Valley, and parts of Solstheim from the Dragonborn DLC. You'll need all of the game's DLCs installed to run it. Cave interiors are now green and temperate, and creator sa547 has gone to a lot of effort to tweak weather effects, swapping snow for rain storms, and adding tropical climates in certain areas.
They've also tweaked game sounds so that you will no longer hear rushing winds coming up from the south—instead, you'll get ambient jungle noises. Skaal villagers in the coldest reaches of Skyrim will now wear light clothing instead of the layered, heavy furs from the base game.
Creator sa647 says that a version of the mod that works with Skyrim Special Edition will be coming "soon".
Judging from the 60+ screens on its Nexus Mods page, along with the comments from players, sa547 has done a good job adding tons of detail without affecting performance drastically. They've also included in-depth installation and compatibility instructions to make sure you can get it going—that's on the mod's main page.
As the comments below point out, there is already a long-standing tropical overhaul mod for Skyrim—if you're interested, I'd have a look at the screenshots of both and take your pick.
Here's a fly through of the Project Rainforest-modified world: