I live about 55 minutes away from TDR so I've been there many times (maybe something like 13?) Here are my tips for those overseas that are considering their journey:
1. July/August is very very humid and miserable in Japan as well as obvious school age child peak times. It's best to be avoided, especially for TDL. I've had good luck with February and September.
2. Always check for when Japanese national holidays fall. On national holidays and associated "3 day weekends" the parks will sell out. Those who purchased tickets in advance will be allowed into the park but no tickets will be sold at the gates. Those people wishing to enter on a sold out day are queued in the parking lot until 5 pm when ticket sales will renew. Even if you do have those advance tickets you will be packed in like sardines, believe me. I've been there. (and this is coming from someone that's done 3 spring breaks at WDW)
3. Stitch, Marie, Mickey, and Pooh are the top characters in Japan and have the most merchandise.
4. The newest attraction at TDL is "Monsters Inc" and at TDS is turtle talk. Nice for the kiddies but really nothing unique to either park and the wait times are painful.
5. BUT the Japanese are not all that fond of roller coaster type attractions (jet coaster is the term used in Japan) so on moderate to low attendance evenings 6pm onward you can often walk on space/splash mountains, grand thunder, and raging spirits.
6. Popcorn is "serious business" and a ritual at TDR. You better believe I have been hoarding the same popcorn bucket since 2005 for refills as it has the larger capacity. The Japanese map will have a popcorn flavor guidemap but the English map does not. The flavors sometimes vary and those I can remember now are: honey, chocolate, salt, caramel, curry, soysauce and salt, cappuccino, strawberry. When in Rome...
7. Another Japanese ritual is buying souvenir cookies, rice crackers, and so on for your coworkers. Which usually come in tins and boxes that no one wants to lug around all day. The shops are completely swamped at the end of the day. SHOP. EARLY.
8. DisneySea has always been marketed as "for dates" which is why it's heavy on the elegant and adult themes and sparse on the kiddyfare excepting the little mermaid area at this time. The 10 and under set might get antsy in the pantsy. (Or perhaps this is just my 1986 Epcot trip bias coming into play).
9. This may be an odd one. But when purchasing clothing items in Japan at the parks note that Japanese sizing runs smaller than western sizing. Also the parks generally only carries one size: M. The female "M" is generally a US size 6~8. The male "M" is S in US sizing.
I hope these were somewhat helpful. Any questions, feel free to ask.