The National Book Tokens (UK) guess the books competition is back. This kept me absorbed during down time for days last year (I guess I'm slow!).
In the interests of disclosure, you can sign up so it saves your progress, even if you're not in the UK, but they do send a newsletter every now and then with cool prizes that you can't win if you live abroad. The puzzle is really fun though.
ETA again: Hah. I finished it. Either it was a lot easier this year, or I got smarter :) Some of these are exceptionally hard if you don't have British English or some near variant as a native tongue (Red Hot's post about the Milk Float got me thinking about this.) And one is near impossible unless you're very familiar with London. So I'm putting a couple of clues in a spoiler here, but even those are gonna be just hints.
Also when you click on a clue, you can click again on the thumbnail by the answer box to enlarge the image.