
90 minutes should be plenty of time to see the lagoon and the beach. If the icebergs aren't on the beach in summer, then go on the boat ride instead. Here's the link to the boat tours of the lagoon: http://www.jokulsarlon.is/
The boats are those amphibious "duck" things that they have at theme parks and such in the US, so they're not an intimate experience at all. The Zodiacs might be better, but they require wearing a rubber safety suit. Just FYI.
I'm not sure where we're going next...I want to go back to Japan, but I'm not sure that's in the budget for next year, so we'll see!