During the summer, a lot of places have an outside seating area, this means there isn’t as much of an issue of being denied entry into the restaurant. Now, outside dining tends to be more casual than inside dining, but this doesn’t mean you can let the dog roam around. Remember, it’s hot out there, so have the dog go under the table if you can or at least somewhere in the shade. Sometimes you need to lay the dog down under a tree or something and you should hook the leash around it, especially since you’re going to be eating and won’t always be able to use a command! I would always bring a carabiner to help have a hook for the leash.
Sometimes you are seated on a paved surface, don’t forget, pavement can get very hot, very fast! I also recommend that you bring a blanket for the dog to lay on and water and a water dish. Sometimes they will have bowls and water you can use, but I wouldn’t assume that they would (but if you forget, definitely ask someone, you never know). I know I may be asking you to bring a lot, but I suggest you make a “summer kit” and just throw it in the car. There are also a lot more distractions, sights and smells than inside a restaurant. Different smells in the air (especially if there’s breeze), birds flying around or hopping across the ground, and if you’re by a road, there are cars. There should be no issues but I would be aware of all this and I hope you enjoy your meal!