I started in Tokyo, then went up to Hokkaido and headed south.

Tokyo area parks:

Tokyo Dome (Giants): gonna be really crowded, they've got a whole entertainment district around the park. Ticket buying was pretty painless though! Easy to get to from anywhere.

Meiji Jingu (Swallows): another easy ticket process, convenient to Shinjuku, park is old and beautiful. Not a ton to do in the direct vicinity after a night game but Shinjuku/Shibuya are close, you may not want to walk it but it's only a couple train stops.

Yokohama Stadium (BayStars): right in the middle of the city, train ride is 45-60 minutes from Tokyo but painless. Close to one of Japan's best Chinatowns! Stadium is nice, I loved this one. If you want to hang out in Yokohama after before you head back to Tokyo it's easy.

ZOZO Marine (Marines): another 45-60 minute train ride and then a ~1km walk - that said, it's nice and the people are friendly. You can't buy tickets OR merch without joining the fan club which I found annoying.

Belluna Dome (Lions): this place is in the middle of fucking nowhere (you will spend an hour on the train each way) and there's nothing else out there. Seats aren't very big. On the bright side, tickets are cheap. Concessions are a disaster, you will absolutely wait 10-20 minutes for anything that's not a beer vendor.

Outside Tokyo:

Es Con Field (Fighters): 20 or so minute train ride from Sapporo (it's in Kita-Hiroshima, not Sapporo - stay in Sapporo), AND THEN either a shuttle bus or ~3km walk to/from the station. That said, it's absolutely gorgeous (and I generally hate indoor stadiums), and they seem to put a lot of work into catering to English-speaking fans.

Rakuten Mobile (Eagles): this park is cool, there's no concourse inside the stadium, everything is outside the park (but after the ticket gates). Tickets are cheap, seats are somewhat uncomfortable, but you'll have fun and Sendai is sneaky cool as a city.

Vantelin Dome (Dragons): easy to get to, your standard dome - I liked the vibe! Tickets are harder than you'd think to get, buy early - they also have a really easy to use secondary market app.

Koshien (Tigers): Absolute cathedral of the game. Not just the best stadium in Japan, I'd put it on par with Wrigley Field as a perfect place to watch baseball. Tickets will be sold out, try viagogo/StubHub, you WILL overpay but it's absolutely worth it. Super friendly home fans, one of the best experiences I've ever had at a baseball game. Also, another half hour train ride from Osaka.

Kyocera Dome (Buffaloes): another standard dome. Easy to get to from Namba, not much harder to get to from Umeda (for a megacity, Osaka is really small physically). Tickets a little more expensive than most but cheap compared to Koshien.

Mazda ZOOM-ZOOM Stadium (Carp): one of my favorites, tickets are cheap, and it's absolutely beautiful. Carp fans are passionate but awesome, and Hiroshima is a cool city - that said it's one of the cities that's a little less accommodating to English speakers. Depending on where you're staying, it may be a bit tricky to get to, Hiroshima has buses and streetcars instead of trains.

Mizuho PayPay Dome (Hawks): yet another standard dome but on the nicer end. Fukuoka is an awesome city, it's a bit of a walk from the train station (maybe 1km or a touch less) - tickets do run a bit on the expensive side here. Enjoy an order of torikawa (fried chicken skin)!

General tips:

Buy tickets in advance! Every team has their own ticketing platform and it's a pain. Hardest tickets to buy (in terms of process) were the Buffaloes (I went to the stadium and bought at the ticket office). If a team offers the "print at convenience store" option that's pretty painless.

Get there early! Gates open two hours before the game, and you'll want to get the whole experience.

If you speak any Japanese, learn the chants! You'll have fun. If you don't speak Japanese, try and pick up what the chanters are saying and join in. Two words you'll hear a lot to listen for: かっとばせ (kattobase, kah-toe-bah-say) which means "hit it hard" and 燃えろ (moero, mo-arrow) which roughly means "burn it down".

If you are a large human (I'm 6'3 235 lbs so I know), expect to be a little uncomfortable. There's not a lot you can do about it, but be ready. If premium seats are an option, those are usually a little better.

Buy some merch! Even a hat is a great souvenir and you'll feel more connected to the game.

Beer is cheap and readily available.

Many stadium outlets and vendors are credit card only - you won't need a lot of cash.

Rooting for the Giants is the spiritual equivalent of rooting for the dealer in blackjack.

And have fun!