Aliens can be a lot of fun, although most of them end up being just townies or interesting neighbors in my game. I think the most played ones in my game are always space changelings, aka results of abduction.
http://i.gyazo.com/5b1bdc1a55dda8fb5f757489e2c8936e.png
This corner of the Oasis Springs is mostly alien builds (not all, of course), so figured I'd just take a quick grab of that and then list build-and-creator names so you can look them up in the gallery if you're interested. Modern Alien House NoCC by KaruPlays, Sixam Spaceship by mszar7, AlienHaus by Daniel_SimFansDE, Eco Alien Starter by gothkittymimi -- basically if you're looking for lots, just do a search with 'alien' and you'll find amazing amount of great lots for aliens! I don't use custom content, so all of these houses are cc-free as well.
I have aliens who wear just alien outfits and are adamant about being who they truly are, then I have aliens who have become sort of hybrid, still non-human skin tones and hair colours, but otherwise dressing and looking like humans, and then of course the ones who wear a full disguise all the time. Also some ghost aliens at Spaceship, died in a ship crash. I also have a scientist who cloned a couple of aliens, and they live with her.
Enjoy the aliens!