You can't get sides with a CS credit unless they come with your entree or combo meal, if that's the case you would have to use a snack credit (or pay cash) for anything extra you want.
If you eat ONLY the muffin or danish and don't get a "meal," you may be able to get a continental breakfast at a bakery or food court with a CS credit, the number and types of things you can get may vary, so be sure to ask before you pick them out.
You don't get 1 snack per day. On the basic plan you get 1 snack credit for each night you stay. If you stay 5 nights you have 5 snack credits, and you decide where and when to use them over the course of your stay. You can use all 5 of them the first day if you want, or you can save them and use all 5 on the last day, it's up to you how you want to use them.