We're making a trip to Iceland this coming April (isn't it great that DIA is getting more international flights?), but this will be the first time there, and we aren't doing a layover en route to other parts of Europe. Even in April, it looks like lots of the countryside and waters are available for hiking, skiing, Golden Circle tours, whale watching boats, etc. We're still planning, but it looks like with just a week, we won't be visiting the eastern part at all, but are considering some of the things already mentioned. We're also weighing renting a car or 4WD truck versus signing up for guided tour excursions.
You may have noted that Icelandair gives you the option of paying extra for their meals, or bringing your own food for the flight. I read in the Lonely Planet guidebook that you can visit a place where they produce Rotted Shark, the local delicacy, and have a taste of the spongy, putrid shark meat, then go out back and see the strips of shark hanging to cure, covered with flies. I'm a fairly adventurous eater, but might pass on that.
This will be our first Icelandair flights, and we've generally used British Airways from Denver, with flights in the middle of the week cheaper than those on weekends.