bristol societies are great, and there's a variety of societies that are very active. These range from course societies and fundraising/charity-based societies to Ethnic societies and whacy ones like Cheese society or Harry potter society!
Personally, my course society (electrical engineering) was pretty much non-existent as the exec were all final years...although i think this is an odd one out. most other course ones are actually pretty good - they have parent schemes where 2nd years mentor and support a handful of first years each!
Spor societies are plenty as well but most expect quite a bit of membership fee, and for those that would be using the Tyndall Avenue sport centre, you'll have to get a sports pass on top.
Also freshers week is pretty good - lots of things going on, although most of them involved lots of drinking...if you're like me, it'd take a little bit of time getting used to. But generally it was good for socialising and getting to know people!