Whenever I have a spare moment or am bored, I just drop down and do a few burpees (when nobody is watching, often in my underwear).
Also doing some plank exercises after the burpees seems to help. Between those activities and some basic heaves and pushups, I find it's enough to keep me fit.
The gym really bores me. Often I try to make the effort and turn up at the gym, only to leave 10 minutes later. Short attention span