You cannot trade CS credits for TS. Sometimes they allow it the other way around, but they never allow trading up.
A combo meal is a meal which, on a CS menu, includes the entree and side. (They must be listed together on the menu. For example, Pizza Planet has a "meal deal combo" which includes a pizza and salad.)
There are a few exclusions, but they are extremely few. The 2008 Dining Plan FAQ thread at the top of this board has a list of some that have been reported by posters.
I believe there are items at Goofys that may be had for a snack credit. However, you must use the credits before midnight of your checkout day, or they expire.