Initially, I planned on doing the Blue Lagoon on the day that I drive from Vik back to Reykjavík (11 am or 12 pm entry time) and then we'd spend the remainder of the day exploring Reykjavík with an overnight there. But I've read that many people did the Blue Lagoon on their last day in Iceland, before their flight home.
So now I'm torn, should we do the Blue Lagoon on our Vik -> Reykjavík day as originally planned, or skip it that day - instead allowing the entire day for exploring the city and save it for our final day in Iceland before our flight home? Our flight departs at 4:45 pm.