I use an activity tracker - the Garmin Vivofit, but there are loads out there on the market. It really depends on how whizzy or basic you want it to be - you could get a pedometer very cheaply, or you could get one that tracks activity down to your sleep patterns.
I tend to use mine to keep tabs on how active I have been during the day - so generally just the number of steps, though it also tracks calories burned based on my height/weight etc. It also has red bars that come up if I've been sat on my bottom for too long!
I found this review of some of the kit on the market at the moment:
http://www.wareable.com/fitness-trackers/the-best-fitness-tracker Really it's just a case of working out what you think would suit you the best. I personally would highly recommend one. I didn't think I would get a great deal of use out of a tracker but thought I would try one out. I love it now and would hate to be without it!
