Went to that Science fair at South Kensington (although I wouldn't call it a fair, since it was pretty much a full-fledged museum). I also went to that Computing Employment fair that my college had 3 years ago. Was fun, got a lot of freebies and some tryouts for the Royal Navy (on the computing side of things) but never got round to using them.
The main one for me was my first trip to Eurogamer Expo last year. I went to 3 out of 4 days and it was a blast! The first one I went alone as I wanna experience how being in a gaming convention is like. It was epic! Got to see upcoming games, tried them out, got LOTS of freebies (from a free OnLive console, game demos, T-shirts, merch, a plastic ocarina from the Zelda people, discount vouchers, free magazines and more). There's so much things to do and 8 hours isn't enough to fully experience it. The other 2 days, I went with friends and it was more of an impulse buy for them as they saw me posting pics of stuff on Facebook and they all went "Dude, we should go tomorrow!". I just booked myself a Super Pass, meaning 10-6PM on all 4 dates this year. Have to say it was my first gaming convention, and it's been memorable!