I am actually really considering Harvard Extension since I have family that lives in the area. How hard was it for you to get in? I currently have a 3.3 GPA, but I was told I can't apply until after graduation in May.
I didn't check SDN for some time, so sorry for the late response.
I called to ask about how hard it would be to get into their HCP program. I spoke with Dr. Fixsen. I have no clue if he still runs the program, as that was over 5 years ago. He basically asked a few questions about my grades and activities in about a 2 minute span and said I would have no problem being accepted. So I applied and was accepted shortly. I had a 3.7x. GPA and was working full time, so our experiences may differ.
Nonetheless, you don't actually need to be in a formal program like HCP. You can just take the same courses piecemeal. HCP only guarantees you a committee letter at the end if you maintain certain grades. You don't need a committee letter (though it may help) when applying to medical school.
There is a Harvard Extension thread in the Postbacc section of SDN. I can answer some questions, but they are at least 5 years out of date. You'll want to verify anything I tell you about HES.
1. It's exclusively evening courses, at least for the core science stuff that I took.
2. I don't believe they have Spring enrollment, as the courses are structured in a particular order. They may have summer enrollment for an accelerated track.
3. There may be some undergraduates there, but most of the students are postbaccs and career changers. The Harvard College students don't take these science classes, but the instructors are the occasionally the same (ie. Dr. Tucci for Gen Chem).