We were able to circumnavigate the island in 8 lazy days, and were sick of volcanic rocks and waterfalls by the end of it. That's sort of what they have on offer, along with some odd architecture. English is spoken more than you'd suspect, but like anywhere, they appreciate the attempt to mangle out a greeting in their language.
We took the rental car off-roading in the NW peninsula (Hornstrandir?) for 3 days, don't do that. It's rugged as hell and there is nothing there.
Get very clued in about when the northern lights run, cross-referenced to the chance of overcast -- they put on one hell of a show up there if you can align things.
Renting the Diesel Subaru outback was money well spent. We literally did the island on a single tank, and it handled the gravel roads like a champ.
There is a less touristy lagoon in Myvatn. It was also blue and very pleasant and only like 15% of the tourist load.
If you do have a car, prepare to be hit up by stoned backpackers looking for a free ride when you stop at a market or restaurant. Those clowns are everywhere on the southern half of the ring road. Completely absent on the northern half.
AirBnB can save you a few coins. Many of them serve dinner as part of the deal. While we will never know for sure, I'm certain that it was a rural Icelandic AirBnB where we ate green salad with horse. Chewy. 2 stars. Would not order again.
Skip the reykjavik mall except for stocking up on strange supplies or snacks. Don't be tricked by the mexican food place. I can't explain what I was thinking. There are plenty of small grocers along the ring road, route 1.
The puffins are rad, but bring a long lens, the tour buses hog the best spots and you'll have every freaking Ansel Adams out there with his giant articulating tripod arrangement blocking the view and trying to get that perfect shot. God those guys annoy me, I wish I had shoved a few off of the cliffs. It would've doubled viewing capacity of the nesting grounds.
After doing that, there are restaurants that serve puffin. We were there late in season so I didn't get to troll my wife who is an aspiring photogue herself by ordering an 8x10 of roasted photogenic creature with potato salad.
It's windy as heck, not terribly cold, but layers are a strong choice. I did most of iceland in only the windbreaker, but there were 2 days where I was happy for the fleecy liner.
You gotta do it once and say you did.