Each user has an individual user RSS feed, so you could use that to subscribe to their user activity. I found it on the lower right corner of the user profile page. My feed, for example, is:
You can also subscribe to RSS feeds for all recent questions at http://homeschoolhelpdesk.com/feeds, and you can find tag-specific feeds on the various tag pages. Here's an example tag feed for getting-started:
There must also be feeds for answers, but I'm still looking :)
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