Not trying to be funny, but why not just take the long way home from work everyday? No need to buy anything, no extra parts, and it'll simply take extra time (which unnecessary drag on the bike would do anyway.)
Why spend time/money on making your bike less efficient? You can just be a less efficient you. (Also, I'm sure folks on the roadie forum have posted lots of methods for interval training and using the "wrong" gear to enhance the workout benefit of a training ride...)