I went to Harvard Extension School. At the time, about half the classes were shared with the engineering school's undergrad program and those tended to be the most fun courses with projects that I mentioned previously.

In many classes I chose what project to work on and I chose to push myself to work on difficult projects (e.g. I could have chosen to implement a basic stack machine with a small instruction set, instead I chose to implement an ARM CPU, which lead to many overnighters to meet the deadlines; the 3D game was my choice as well - I could have picked to implement an easy visualization in opengl).

In all honesty some of the intro courses were not worth the price tag, but the advanced ones I'd consider taking again just for fun. If your employer covers the tuition, then I'd say that any course taught by instructors at SEAS (Harvard's engineering school) is absolutely worth taking.