Summer in Tokyo is HUMID. Mushi mushi, mushi astui and atsui. Those are the only variations in weather from approximately July until early September. Here are some ideas to help you deal with the Summer here!
Firstly get yourself an ice cream or icy pole for that walk home. People may give you a look like you have 2 heads for walking and eating at the same time but I do it anyway and it is BLISS. My icy pole recommendation is the famous suika bar. It looks like a slice of watermelon and tastes like a real one too except for the chocolate pips which add extra joy to the whole experience.
If you want to get out of the city and head to a nice beach for a swim I strongly recommend that you DON'T go to the Shonan area. Unless you go to Hayama and before the end of August. Do make the effort to go to Izu in Shizuoka, Onjuku in Chiba or around the Wakayama area. In those locations you will find beaches that are a lot nicer. Also any of the beaches in Okinawa are a must see and compete with some of the most spectacular beaches in the world.
I also suggest if you don't have an entire day or enough time to travel far, you could go to one of the many outdoor swimming pools Tokyo has to offer. In my opinion, the outdoor pool in Shiba koen near Tokyo Tower is best. No need for those hidious swimming caps or goggles but only downside is no sunscreen or tanning oil allowed. The tanning oil I understand, but I'm still trying to get my head around the no sunscreen rule. Don't even try to sneak it in or apply it in secret as I've witnessed the lifeguards inspect the pool water and walk circles around those who bake their bodies religiously and they actually sniff for oil. The pool in Shiba koen is seconds from the station of the same name on the Mita line.
You could also come and visit me at Pretty Things in Komazawa for an iced coffee or drop in at Bowery Kitchen for some Summer menu deliciousness . I was quite pleased with this dish I had today on my lunch break.
Enough from me. Enjoy the humid/mushi mushi/mushi atsui Summer in Tokyo and to those of you back home, keep warm and be grateful you don't have to deal with untamable hair and a sweaty back for a total of two months or more. :/