We just got back from a Japan trip that included two days at Disney Sea and one at Disney Land. First of all, let me just say that Tokyo Disney is just absolutely amazing! Disney Sea is a beautiful park and even though it's crazy crowded it's fun and friendly and incredibly clean and well organized. You can see pretty much everything in one day but be prepared to put in a lot of steps!
As far as planning, we were there when you still needed to book a timeslot for the standby lines in Fantasy Springs so we were able to use a combination of DPA and reserved standby to get in all the rides in Fantasy Springs. If you're willing to buy DPA's for all the rides you should be good, otherwise you're looking at hours in line now that standby lines are open to everyone.
If you're not staying onsite, you also have to plan for pretty massive crowds at entry. We arrived at 630am on the first day for 9am open and there were a few hundred people ahead of us. But we were among the first through the gate and were able to get all the rides we wanted that day. The next day we were more chill and showed up just before 9 and it took almost an hour to get through security and into the park because of the crowds ahead of us. You can check on DPA availability on the app to see how quickly they sell out in the days before you go in order to figure out how early you need to arrive. But you have to be there at least two hours early if you plan to rope drop the big rides without a long wait.
And speaking of purchasing DPA's, we couldn't link our credit cards to the app until we were inside the park. Then our bank had to send a text in order to confirm the purchase and our eSim wouldn't allow us to receive texts so we had a few anxious moments watching the time slots tick away while we switched back to our Canadian sim card and turned on roaming in order to do the transaction. It worked out in the end, but I wish we'd sorted that out before entering. Having the credit card linked was also great for pre-ordering dining. The Snuggly Duckling wasn't taking walk ins when we were there so I'm glad we pre-ordered in the morning.
We also noticed a lot of characters roaming around that you don't often see in American parks. They really seemed to like the cutesy animal characters there, like Thumper from Bambi and Marie from The Aristicats. And a big part of the fun was seeing the Japanese kids all dressed up in group themes, posing for endless pictures and having a great time. Disney Sea definitely felt like the more teen/young adult park while
Disneyland had more families with young kids.
Hope this isn't too much info for you. Have a great trip!