One of the highest roi investments I have seen is restaurant business. All you need is good price and a delicious original seasoning. Profit can reach to 100% over each meal besides the drinks that are more expensive than at supermarkets.
At beginning it can be difficult because there is the initial investment (stove, freezer, miscellaneous machinery), but once you pay it, the profit is huge, of course if your business has quality.
This is a good business but how much would be the capital and how long would be the expected ROI?
I'm not into fine dining or restaurant business but this is also an interesting industry to hop in.
The capital needed and the roi time will depend on your structure and the service being offered.
There are many variations that must be studied and calculated cautiously: it can be a simple wagon (hot dog, salty food, candies) or a rented building or your own building (what is better as it's less one cost already). Then you have to check what else you need to buy (stove, freezer, tables, chairs) and calculate how much it will cost, what meals you will offer, if there will be any desserts, how many customers you expect daily...