I can relate to the troubles with the summer heat--I get seasonal depression in the summer and it heightens symptoms of other stuff too.

people here have already posted some tips for the heat so I can't offer much more there--but one thing that I've found helps with my mood when I'm stuck inside all day, personally, is having plants around. I've placed them near my bed so I can enjoy looking at them even when I'm lying around in bed all day and feeling low from the weather.

some plants really low-maintenance but enough to keep you moving at least once every few days, when you need to water them. the fact that the plants I have do well in sunny/slightly humid weather also helps--if I can't enjoy it, at least I'm helping create something that does.