Hi MissTammy,
I have been to both Munich area and Heidelberg area more than a few times in the last few years, unfortunately for work, though. Heading back to Munich area in June (A day after I return from DW with the family). Munich is nice, as PP mentions Oktoberfest is not the best time to see Munich at its prime, esp last year during the 200th year anniversary. Local coworkers say most arent even Germans. I went during what my German colleagues call "Italian week" in 2009. It was fun, but was there last year but stayed away. Not sure how packed your days will be but if you are left to somewhat schedule your own events, check out Mikes Bike Tours. The owner was originally from the US and caters to English speaking tourists, and I know some folks who have done the tour and liked it, although I didnt due to the weather. Also, I have taken the Dauchau Concentration camp tour as it is close to the Munich airport where I rented the car. But I saw many tour buses coming in, so they obviously cater to large crowds too. It was extremely moving. You can choose a self-guided audio tour (English) which I did, which was 3.5 Euros, plus I think it was a 10 Euro deposit which you get back when your return it. A guided tour is also avail which you would go with a group, but they only do one or two tours per day in English.
As far as Heidleberg, I work in Mannheim which is a quick train ride away. Mannheim is an area where there is a lot of shopping, but prices are high (norm for pretty much all of Europe I found). Heidleberg castle is pretty cool. It's a somewhat challenging walk up to it, so wear comfortable shoes. It is free to walk up to it and around it, but there is a charge to enter the courtyard and into the open areas of the castle. They were doing some renovations on the castle so not all of it may be accessible. But its a beautiful view nonetheless. The city itself is really cool. A good variety of restaurants, pubs, etc. We went into a pub there that had a tree growing in the middle of it and did some sort of flaming shots. Went with a German colleague who knew the area. Most notable restaurant was where we were served the largest platter of meat I ever saw. Not sure what all of it was, but it was great. I can try to look back at my records if you are interested in the names of the restaurants. There are also many bars which legally sell absinthe, a high proof liquor which is rumored to have psychoactive properties. This appeared to be all the rave for younger tourists.
Either way, I know you will enjoy your time. I have found the local German folks in Bavarian region to be extremely polite, helpful and tolerant of us Americans. If you havent already, I'd suggest learning a few basic German phrases which is even more appreciated as it shows at least an attempt at respecting their culture. I went so far to start taking lessons on Rosetta Stone as I am there fairly often and enjoy it there so much.
Enjoy!!!