Docs are a great place to start. Getting started / installation guides, particularly for folks who can't use "the usual" install method, are easy and obvious wins. Curating existing answers into a categorized
FAQ is useful. Having a site rather than a Githubbed Readme is a win, and gives you numerous options to show off web dev skills. Failing that, you can pop open any jQuery plugin or well-used Rails gem, open up the issue tracker, and pick off an issue or two of low-hanging fruit. It might take you a day or two when it would take someone familiar with the codebase 15 minutes, but they clearly haven't done it yet, have they.
The first OSS project I ever committed to was an online Battletech platform. I found an O(n^2) algorithm via visual inspection of code that one of the core devs had mentioned was a hot spot, investigated why it couldn't be quicker, and ended up introducing a caching layer. (It was 10 years ago, but my vague recollection was that caching the results of the inner loop made is essentially O(n).) Immediate perceptible speed boost. The caching code was an abomination but it worked. (The story of my life, I swear.)