Bónus will be your go-to grocery store as you drive around the Ring Road. Keep track of their opening hours, as they close at 6:00 or 6:30 PM most days. They also have an English website https://bonus.is/en/
Fresh fruits and vegetables tend to be more costly, but there will be plenty of bread, yogurt, meats, cheeses, eggs and pastas to choose from
If you're looking for cheap souvenirs, you can pick up Icelandic chocolate from any grocery store
Gas stations will offer hot meal options that are fairly basic but tasty: hot dogs, soup and bread, burgers. Icelandic hot dogs with fried onions are available pretty much everywhere. For breakfast, skyr, an Icelandic yogurt is an affordable option that's great for a road trip
Buy your alcohol after landing at the duty-free before leaving KEF airport. If all the locals are doing it, you know there's a good reason why.