Vichyssoise (French, cold, leek and potato soup) is very easy to make (just sautee alliums, simmer everything, blend and add creams), and very filling. I like it with garlic croutons. The salt is welcome after a sweaty day, and it's very easy to make vegetarian or even vegan if you like. Since it's served cold you can make it earlier in the day when it's cooler and have it for dinner when it's hotter (honestly making it a day or two in advance makes it even better also).
Cold Somen (Japanese cold noodles with various toppings and sauces) this can be a template for pretty much whatever you like with noodles but cold. The thin cold noodles pair well with a tart soy or vinegar based sauces and all kinds of veggies/meats.
Never forget the versatility of charcuterie on a hot day (especially to avoid making the kitchen hot) - for just one or for a whole crowd, help yourself to sliced and spredable cheeses, crackers, toast points, cold cuts, nuts, fruits, olives, pickles, veggies, jams, tinned fish/shellfish/meat, dips, etc. Remember it doesn't have to be overly formal or fancy, but setting up a plate for yourself will help you feel you've eaten a proper meal rather than just grazed out of the fridge (my very most simple go-to is whole wheat crackers, salami, sliced cheddar, and a sliced apple or some grapes - particularly nice to take your time to nibble if heat has your appetite down also).
Sandwitches and wraps are hot-weather friends as mentioned, and outside of the basics like ham&cheese or turkey&lettuce or roast beef or what have you some nice other cold variants are; tuna salad, egg salad, chicken salad, chicken ceasar wrap, curry chicken salad, club sandwiches, turkey with pesto mayo and mozzarella, bruchetta and others.
This might be terribly American but I do personally like a waldorf salad (apples, celery, and walnuts in a mayo dressing), especially with some chicken added for protein. I also like my family's go-to all-season fruit salad which is just sliced banana, mandarin or orange wedges, chopped apple, and walnuts or pecans - plus anything seasonal that sounds good (no dressing).