Deniz, Ive read weekends are their busiest times. So you will want to use fast pass as much as you can. I would start off with a FP for Winnie the Pooh, yes you will like the ride, its not just for kids. I would then do Big Thunder, I thought it was different then WDW, I could be wrong. I know its different then DL. Space Mt. is like DL. Im not sure about splash, it was cold we didn't go on it. Oh do the Western Train ride, is one to not miss(no FP, but the line moves pretty good). There is no train around the park, due to in Japan, if you have a ride that has "x" amount of track, it considered public traspertaion and they would have to charge guest to go on it. So they made the train a ride and its pretty cool. Great place to get some photos.
As for Tokyo Seas, there is a lot of walking if you want to see everything. There are boats that can take you from the front part of the park to the back, which we didn't know about until our last day or so. Im not sure what I would say is a must see for Seas, because its all a must see.
With in the two parks there are seven different flavors of popcorn, so have fun trying them. I oddly liked the strawberry one. lol