Hello Stewbidoo and welcome,
mid-March might be a little early for hanami, but if not, you probably want to spend some time to enjoy the cherry blossom. There are great spots for that:
http://www.secret-japan.com/forum/viewtopic.php?t=101
Kamakura also has nice cherry trees, and even if it is too early for hanami, it is always quite a romantic destination.
For aquariums, there are two which are really worth the trip:
- Tokyo Sea Park Aquarium : www.tokyo-zoo.net/english/kasai/main.html
Located in the Rinkai Koen, next to the sea (very close to Disneyland), the place is really attractive. The aquarium is great, and there is a nearby natural reserve for birds, where you can watch some wild birds. Even better, there is a possibility to go there by boat from Tokyo or Odaiba - a great journey if the weather is nice - Yokohama Hakkejima Sea Paradise : www.seaparadise.co.jp
Slightly larger, this small island is a little far from Tokyo, but also has an amusement park.
I would recommend the first choice...
Other interesting options could be a helicopter tour above Tokyo. Great by expensive...
If you have enough time to travel outside Tokyo, and apart Kamakura, you could also go to Hakone - a perfectly romantic destination if you can see the Fujisan.
sanji