I know that the movie Avatar and the world of Pandora with the Na'vi people is a controversial one for Christians - and although I enjoyed some aspects of it, would not recommend it - but the concept used is one that is brought to mind straight away when I read your question. They are human-like, but also distinct. There are some ideas you could draw on - they are taller (7-9ft), blue, have a tail, cat like yellow eyes, cat like features, 'tribal' in their dress and customs, warrior-like, ...... ummm.....come on wikipedia what else can you give me?? haha.
Another idea could be to look at different cultures around the world, and emphasise one, or an aspect of one. Everyone dresses and looks differently, and has different world views and ways of doing things.
Then of course there is changing human structure, such as adding wings, a tail, or changing the skin so it is a different texture - scaly, feathers.....?
You could take animal features (eg cat like eyes), or perhaps give them an extraordinary ability, like being able to jump (I'm thinking John Carter here!) or climb.
I think also that just the way people relate to each other can mark out a vast difference between species. Being 'as one'/equal in all aspects is often something that stands out to me as being a common theme in a lot of non-human species.
Anyway, hope I've helped!

ETA: I just re-read your post and you mentioned you wanted them to be able to pass as humans. hhmmmm maybe some of my ideas won't work then! lol