It blows my mind that phpBB doesn't come with RSS feeds for forums. The new v3 is incredibly nice in so many other ways, it seems like it would have been a trivial addition for such a talented dev team. But what do I know.
Fortunately, a kind forum reader dug up a link for me for a mod that did the trick!
There are all kinds of options that can be set, but I thought the most useful (and the most requested), would be if the feed listed only NEW topics posted (as opposed to every single post anyone makes). That way you can only click over if it seems like the thread will be of interest to you.