I do fairly regularly, since about 2003. I used to be very involved in meta-Wikipedia stuff in the early days (policy, mailing lists, meetups, dispute-arbitration, etc.), but sort of drifted away from that once it got more professionalized, and had a Foundation set up and such.
Now I mostly write short (~2-10 paragraphs) articles on subjects that don't have articles. I usually start from a good source, like an encyclopedia of scientists, or a book on 18th-century Scandinavian art, and look at what it covers that Wikipedia doesn't yet. Then I pick one of those things, look on Google Books/Scholar for additional sources (it's ideal to have at least 2 sources for an article), and write a short summary of what I've found on the subject.
A different kind of editing, which I do mostly as a kind of "don't have to think too much" unwinding that I find enjoyable somehow, is various stuff from a long list of filing/curation/sorting tasks that always need to be done. For example, take something from the category listing the thousands of articles that could/should have lat/long coordinates added but don't yet [1], and see if I can find its coordinates (on OpenStreetMap, on official websites, etc.).
Another useful low-key thing to do is to pick some articles that you know something about or have an interest in, but which are less popular (not hugely popular articles like [[Barack Obama]] that already have many editors), and add them to your Watchlist. If you then check the Watchlist periodically for edits to those articles, you can be part of the distributed tiny communities curating the more obscure parts of Wikipedia. This can be sometimes just be noticing when someone makes a questionable edit (whether spam or otherwise). But it can also be positive attention, e.g. if another editor asks a question on the Talk page proposing a change to an article or questioning part of it or asking if something they added is ok, you'll see that on your Watchlist and might have something to weigh in with, which makes the parts of the encyclopedia you're paying attention to feel like less of a ghost town (it can be frustrating when you comment on a less-popular article and literally nobody answers for months... the opposite of the problem you get in really contentious parts of the encyclopedia).
A final really easy thing to do, and probably Wikipedia's most famous cultural export, is to add {{citation needed}} if you run across a sentence making a claim that seems like it really should be backed up, but doesn't seem to be cited anywhere in the vicinity. Even better to find a source and add <ref>Source</ref>, but tagging as citation-needed is still helpful. This is not only for claims that you think are dubious or wrong, since even likely correct claims should have a reference where readers can verify.
[1] http://en.wikipedia.org/wiki/Category:Articles_missing_geoco...
