Wall push-ups are a good way to get more upper-body exercise when you may not have time for floor push-ups, or maybe you're not so great at floor push-ups. They benefit chest, shoulders, arms/triceps, and core muscles. Step back just a little (safely) to "lean into the wall" more, and make the push-ups a bit more challenging. If you need to protect a wall from natural oil from your hands, try wearing non-slip or rubber gloves.
When I go grocery shopping I usually don't buy a whole cart load, but have them packed into two heavy reuseable shopping bags. That way on my way to the car, which is way over to the end of the parking lot, I "curl" the bags halfway up so my elbow is at a 90 degree angle and I hold that curl until I get to the car. I enjoy that nice little extra bout of strength training! :-)
Everyone is short on time and long commutes do not help with this! To fit in some strength training on the freeway, I have created a series of arm movements (one at a time, of course) that I can do while driving. These involve pulsing pushing arm circles etc. Just put on some music and experiment!
Working out on those elliptical trainers at the gym can be fun, especially if you are listening to music or an audiobook on your iPod. But after a while, the routine can get stale. Many people only use their legs and just let their arms "hang on for the ride", but this is not an effective workout.