Skyrim Nexus has an obnoxious amount of mods. I literally spent about 6 hours looking around trying to find what I wanted. Unfortunately what I picked out probably won't work for you, as I was looking for the absolute best without regard to the performance, and I believe several of the mods specifically stated to not even try them without top of the line specs and a whole lot of memory. Plus I don't have access to that computer at the moment to look up which ones I downloaded.
Either way, just browse through the Nexus, you can sort the mods by category, and popularity, so pick a category and look through the more popular mods. Theres quite a few mods that only affect certain areas or types of graphics (like fire animations, trees, etc) that do quite a bit for the overall look and feel of the game without requiring much.
EDIT: I just saw the specs in your sig, if thats the rig you are running Skyrim on, I wouldn't worry too much. You've got me matched or beat on everything except I've got 4GB on my 680. I don't know how much of a difference that will make, but I wouldn't worry too much if I were you. I picked up pretty much every graphical improvement mod out there and the game runs as smooth as it did un-modded.
Also, do yourself a favor and pick up the Nexus Mod Manager. Its a free download, but you have to make an account. It is sooooo worth it though.
If you don't use it, you have to do the manual installs, which usually involves a lot of manually moving files around and placing them in the correct place, making modifications to .ini's, etc. Then you have to figure out the correct load order for the mods on your own which can be quite a pain in the ass as it sometimes involves a lot of trial and error, lots of desktop crashes, corrupted save games.
With the Nexus Mod Manager, almost every major mod has a link on their download page to download using that, and it allows you to activate/deactivate any mod at will, and it can auto-sort your load order for you. If you only get one or two mods, its probably not that big a deal, but its no skin off your back to use it, so why not?
EDIT EDIT: Bah, I tried to go to the Nexus site to see if I could locate a couple of the more useful ones I picked out, but its apparently under maintenance. Definitely do pick up the official HD texture pack, as some of the mods require/recommend it in addition to the mods themselves. Its listed as a DLC pack in Steam, but its free. You do have to go to the Skyrim store page and specifically tell it to add to your game or it won't download it.