What you can do as opposed to what you are willing or unable to do usually comes down to category of wash, property size, budget and personal preference.
The arrangement of car wash and gasoline on the same site is particularly strong and a convenience store is not absolutely necessary to make this work.
Gas, convenience store and car wash on the same site is even better but the c-store brings more development cost, labor 24/7, shelf stocking items, inventory, licenses, government regs, etc.
Car wash with 2-bay quick lube is still viable. Figure $275,000 investment and 8,000 SF footprint.
Car wash with drive-thru express coffee still works in certain markets. Figure $225,000 investment and 9,000 SF footprint.
Car wash with express detail is viable. Figure $250,000 investment and 15,000 SF footprint.
Contract postal units (CPU) have worked at car wash sites. With pending cutbacks at USPS, a CPU or print and ship franchise might be a good choice in certain markets. Figure $35,000 investment and 500 SF footprint.
Sometimes I wonder how well a neighborhood fruit and vegetable stand would work as an add-on business to a car wash. I like this idea better than a dog wash.
Hope this helps.