You have mentioned that you answer some questions on Mathematics. Already this can be considered as giving back to community, even though it is on another site.
This should be mentioned with the caveat that many MathOverflow users have strong opinions why Mathematics is actually a bad site. Probably anybody who spent some time around this site has noticed a few mentions about this - mostly on meta (And it goes also the other way round, there are many people with not very good opinion on MathOverflow.) So you have to decide for yourself whether or not you consider contributing to that site a useful thing to do.
Mathematics has much higher traffic than MathOverflow and an average question there is on lower level than most questions here, so it is a bit harder to find interesting questions there. Since you are most likely interested in more advanced staff, it probably would make sense to add a few areas you are interested in to your favorite/watched tags and simply check only questions in those tags. Or simply bookmark a link to search which returns questions in the tags where you are most likely to add answers. (Notice that search results can be sorted by recent activity, or you can choose the unanswered tab which is more likely to contain questions where answer is missing.) And there are also other ways how to find interesting or more advanced questions, you could find a few discussions about this on Mathematics Meta.
If you feel that you want to give something back to community (or "do something in return" as you have put it), answering is not the only way to do that. There are many other things which are useful for the site and some of them do not require too much from the user. Even though it was posted on another site, at least some of the answer posted on Mathematics Meta in a related discussion apply also here: How can non-experts best contribute? (Although one should keep in mind some differences between the two sites.)
- You can help the site with editing. Even if it is just correcting a typo or helping user with MathJax/MarkDown. Occasionally replacing a dead link with a working one.
- Retagging questions which are missing some tags or are tagged incorrectly is also useful. If you want to help with this, it is good to be familiar with Frequently asked questions about tagging on MathOverflow. I have wrote a bit about edits and retags in another recent answer of mine.
- Flagging what needs to be flagged. (For example, it only takes six spam flags to remove a post. So if you notice a spam post be sure to flag it - you will help to make space for on-topic questions on the site.)
And there are probably many other ways how you can do something useful for a site.