<i>Implement a “useless” algorithm like a bloom filter or a skip list...</i><p>This a hundred times! The majority of us are probably working on web-related stuff that usually does not need a lot of CompSci on a daily basis.
Picking out an fancy algorithm or a niche data structure and implementing it in a language you're just dabbling with can be a great way to learn a bunch of new stuff in a couple of hours. Set two hours to do this once a week and you'll be amazed at how fast you end up understanding new concepts.