A very simple way to add a little more activity to your day is to take the stairs whenever possible. Even if you work in a very tall office building and don't want to take the stairs all the way up to your floor, you can get off the elevator a few floors short of your floor and walk the rest of the way up.
You'd think that the stairs would always be close to the elevator but often that's not the case. Sometimes just finding the stairs will get you a bunch of extra steps.
If you're up to it and want to really to make it a workout, take two steps at a time or run up the stairs. That's sure to get your heart rate up quickly.