Im in Japan right now, lol, and leaving tomorrow :(. Here's my take:
I don't know where all you guys are from and/or your incomes, but Japan (and Tokyo above all) is crazy stupid expensive. Like, you want to cry yourself to sleep expensive. So you should expect everything to be x3 more expensive during the Olympics.
In Tokyo I stayed in the Asakusa neighborhood. It's pretty far off from the "'main" tourists spots and downtown, but it's way cheaper (both accommodation and food around here). Also, Asakusa itself is pretty touristy (Sensoji temple) so it's still a good, quiet place to stay.
I've noticed the language barrier is a thing. It's a mess. Barely anyone speaks english. The good thing is, most subway/metro guards do, so if you're lost just ask them. I'm sure they'll have more english speaking people around the games, though. Get the metro pass (there are different ones, bc different lines, some private, yada yada). Get JR pass. You can travel between cities and locally (ocasionally) cheaper that way.
Places I recommend you visit:
Kamakura (50 min near Tokyo)
Kyoto (2 hour ride or so in bullet train; w JR pass might be longer)
Near Kyoto: Kurama dera and Kibune (great onsen in Kurama-dera)
Nara (although you're not missing much)
Hiroshima + Miyajima (go Miyajima in the morning ~ 10 am; then Hiroshima for the museums before 6 pm)
Osaka (just for 1 day; not much to see. Make sure to go on a weekend, it's a big party city if youre into that). Himeji is close to Osaka, nice castle.
Other things:
Everything opens late (11:00, sometimes 12:00). There are a few places opened earlier offering breakfast. Also, everything closes early (6 pm, sometimes - specially museums, castles and such, close much earlier)
There are almost no places to charge your phone (only some Starbucks and like tourism information offices and internet cafés). Buy a battery, charge by night. Also rent a pocket wifi or buy a sim card, it is life saving. Keep em charged.
7/11, Family Market are open 24/7. Most of them have toilers too if you need them. 7/11 is cheaper and has better quality food imo.
You can drink alcohol on the streets.
You can't smoke/eat while you walk. There are smoking designated areas (some places, bars specially, allow smoking indoors).
Partying is expensive (much more so for straight men; think 1500 yen for women - 3.500 - 4000 for men).
If your LGBT, go to Shinjuku. There's a bunch of gay bars there (I recommend Aiiro cafe and Eagle; nice, friendly open gay bars). There's a lesbian bar, too (women only).
Speaking of Shinjuku: if you're into jazz, go to Shinjuku Pitt Inn. Nice jazz place (but $$$).
Pharmacy's and mini markets have condoms if you need them. You can also buy lube but only in pharmacy's (they might laugh, just ignore them). If you need bigger condoms, there's a 5 floor sex shop in Akihabara.
Akihabara: go to Super Potato if you like retro games. Also, don't buy things there before checking Nakano Broadway. They sell merch and retro games there, too (and cheaper). I also found Mother 3 there (unlike Super Potato). Go to a maid café (it's a weird experience; they're pricy though and charge a cover charge and additional ones if you stay longer than one hour). Also, main character's merch are more expensive (like, Pikachu stuff will be more expensive than say Chansey).
Shibuya: there's a nice thrift shop (also, cheap), near Forever 21. I think it's called Comme des Garçons. Anyway, clothing isn't as expensive imo. Go to hachiko, the crossing. Go to Loft store (largest one in Tokyo), if you love stationary and planners (they sell the cute Earthbound 2 Hobonichi Techo there). There's a cool music cd store I cant remember the name (they sell city pop music as well).
Subway ends at 00:00. If you're far, never take a cab. It's insanely expensive. Just make it till 5 am somehow. You can sleep at a Burger King if you need to. Party all night or go Karaoke (you can sleep there too, but its more expensive lol). Or hook up and get an actual bed! Yay!
Go to Team Lab Planets. Trust me. Amazing experience. You can look up a video but I'd suggest you don't. Just read about it.
If you need tickets for Ghibli Museum (they go out fast; try govoyagin. 100% reliable). Also be aware the museum and other places could get maitnance.
Food:
Try okonomiyaki (both osaka and hiroshima versions)
Tonkatsu
Takoyaki
Sushi (go to Tsukiji market in the morning)
Ramen
Onigiri
All thing matcha
If you're vegetarian or vegan, download the Happy Cow app. There are veggie friendly places, but not cheap.
I'll add more things later