Are there any suggestions for fast/light/cheap lunches? Like a sandwhich place, or a salad or noodles place.
Praire Berry Winery has a nice selection of sandwiches, but they won't be what I'd call cheap.
I'm all for picnic lunches. Sylvan Lake is a wonderful picnic spot. Rapid City has 2 WalMarts where you can buy reasonably priced supplies.
To go along with what slrtravelplanner said, there is a grocery store in Hill City if you wanted to purchase the components for a sandwich. We also found that picnics work well because sometimes you find yourself in an area with no restaurants at mealtime.