You've checked the forecast and even go so far as to stick your head out the door, and you realize no one will be happy in that heat. Deciding to stay indoors during hot days is perfectly fine, and you can work on training activities indoors instead. Your goal on hot days should be to keep your indoor-bound dog happy and engaged because boredom might lead to bad behaviors, anxiety, or depression. Training activities are easy and free since you probably already have treats on hand. You may want to use training devices, if you have them, like hand clickers, but these are not necessary. Simple tricks are also a good way to engage your dog and prevent boredom.
It is hot outside, and your dog is itching to play, or nature is calling. Instead of ignoring your hound's needs, take them outside and let them run around for a few minutes while you fill up your dog's wading pool. Wading pools, or kiddie pools, are relatively inexpensive and you can find them at most home improvement stores or box retailers around summertime. Get a small one and designate it as the dog's pool. You never want to fill the pool up too high and always supervise your dog while they are playing in the pool. Pool time activities are great for hot, sunny days but try to avoid the very hottest parts of the day between noon and 3:00 p.m. Aside from the kiddie pool, you will also need access to cool, fresh water to fill the pool and some of your dog's favorite water-safe toys. This is a cooling activity that should have your dog excited, especially if your big dog is a water dog.
Hide-and-go-seek is a fun game for many dogs. It might be easier if you have a second or third person to help your dog "count" while you go hide though.
Your house might not be big enough to get a good walk in with your large dog, but most pet stores are big enough to explore. These stores encourage pet visits, and they are usually air-conditioned.