I really like aquariums - but have never been to the one in Okinawa. I was at the aquarium in Osaka - and it is excellent - out of the ordinary (at least for someone used to places with lots of tropical fish from the Atlantic Ocean part of the world). The closest thing I've seen to it in the United States is the Tennessee Aquarium in Chattanooga. Except - of course - the aquarium in Osaka concentrates on Japan and the Pacific. Here's a link for your friend:

http://www.kaiyukan.com/language/eng/index.htm

Osaka is doable as a day trip from Tokyo. You could have lunch at the food court adjacent to the aquarium. Which - at least when we were there - had some rather pedestrian restaurants like KFC but also a somewhat hokey recreation of an old restaurant street with various kinds of restaurants. We had a decent Korean BBQ lunch there (quite good by "restaurants close to major attraction" standards).

If anyone likes food markets/streets - Osaka has a pretty good one IMO.

Also - you didn't say what days in April - but I'm pretty sure that's when the Osaka Cherry Blossom Festival is. I haven't been able to find the dates for 2013. Perhaps someone can help me out. If you'll be in Japan during that festival - I have only one word - GO. It's great - blocks and blocks of trees and blossoms and food and drink vendors . Very festive - and tons of good eats (a lot of the food is what I would label kind of "exotic" - but almost all of it is very cheap - so you can try things for 100-200 yen and toss them if you hate them). If you can do the aquarium during the day - and the Cherry Blossom Festival at night - well I wouldn't miss that day for the world. Only problem is you'll probably wind up drunk as a skunk after the festival - and might want to overnight in Osaka instead of trying to get back to Tokyo . Robyn