Five nights in Iceland

Because my time in Iceland is fixed, I'm starting to think through my plans there first.  I have five nights.  I'm thinking of spending the first two nights in Reykjavik, then renting a car and traveling out of the city for the last three days.  Finally, I can spend my last day at the Blue Lagoon, which is very close to the airport, before my 5pm flight home.

What I'm trying to figure out is where to go and where to spend the night for the last three nights.  I know I want to do the Golden Circle, and I could probably extend that into a multi-day tour with detours, or I could do the Golden Circle in one day and then go further afield, perhaps to West Iceland or South Iceland.

Of course, another options is to just do day trips and stay in Reykjavik the entire time, but if I'm going to have the car, I like the idea of not having to get back to a home base each night.

UPDATE: I have made my hotel booking for the first two nights in Reykjavik at Central Apartments.