I got some sandbags (one 12.5kg and another 22kg) and they have been really fun to use. Unlike dumbbells that can be grabbed in only so many ways, sandbags have a bunch of different handles on them allowing one to do bicep curls, bent-over rows, squats, military presses, deadlifts, etc. What I also really like about them is that, unlike dumbbells, they are soft and won't dent your floor or make much noise if dropped. Resistance bands, a pull-up bar, and TRX-like suspension systems are great options too, but I find the sandbags more fun and versatile.

But while strength training is great for maintaining muscle tone, I find that in small quantities (below the point of breaking a sweat) it doesn't quite offset that feeling of sluggishness in the body after a long day of sitting. Jump-rope, jogging, and biking are nice because they get your heart really going. But best of all is rowing because it's a full-body, low-impact-on-joints activity with a wide range of motion, and you can vary the intensity easily by adjusting your amplitude and strokes per minute.