I'm curious-- since you've been there before, is your aim to see/do new things, or see/do things you loved the first time around? Or maybe a bit of both.
This trip is actually longer than the first one, so we'll need to figure out where to extend the trip and what sort of activities to do. I'm also not sure what the road situations are going to be like in late spring.
We definitely want to go to the westfjords again, and there's a place open year round to stay there. Perhaps 3 nights this time and not 2.
On the north side, I think we'll base out of Husavik this time since we really liked the vibe there when we were there for lunch last time. (Last time we based out of Akureyri and it was just ok). We'll do stuff around Lake Myvatn again and go to the nice hot spring near there (and likely skip Blue Lagoon, since we did that last time and didn't enjoy it as much). Husavik is the whale watching capital, so we'll do one of those on an old wooden sailboat, since we skipped it last time. There are also some fjords up there that we skipped, too.
I'm unsure what we'll do on the east side. We stayed in Seydisfjordur last time and the hotel was great, but the town is really tiny and it essentially closes down in the off season on days when the ferry isn't in port. I'll see about other places to stay over there, because I'd like to explore more there.
On the south side, we already did the glacier hike, and I don't know what we can do there in April on our own hiking-wise. Iceland's worst weather is on the south coast, so I don't want to bank on anything there.
We stayed in Reykjavik for 3 nights last time and that felt like one too many, but we just wanted to get away last time, so I'll see if there's more we can do. I already contacted the O club to see if they'll have anything while we're there.