Pushups
Last few months, I do pushups whenever I get a chance. Averaging about 120 pushups/day for the last ~3 months. Got a few friends/family interested, so now there's ~25 people doing pushups everywhere (bars, sidewalks, airports etc.). They've set goals of 10K or 50K pushups in a year, depending on their desire/fitness levels.
The results have been surprising, people who could only do 1-2 pushups at one time, are regularly doing 50+ over the course of a day.
It's been a lot of fun, even if it's not the most balanced workout :D