We are renting a 4x4 vehicle.
You don't need one.
Day 1
You've got quite a lot packed into this day. Seeing sights in Iceland can take longer than you might think. I don't think the cave near Hvolsvollur is really that exciting in comparison with the great scenic attractions of the region, and is likely to involve you in a degree of fuss to get the key. The saga visitor centre will be interesting mostly if you are really interested in sagas, to the extent of actually having read a few already, and feel that you really want to go there. The area is particularly noted for its connection with Njals Saga, but this isn't the saga I would recommend you read first, as it is rather long and repetitive.
Eyjafjallajokull & Myrdalsjokull
In what sense are you visiting these? They are ice caps on top of mountains, which you can see distantly from the road if the clouds aren't there, but often they hide. There may be trips available in snow travel vehicles, weather permitting, but if you are going to invest in those you'll need to provide a lot more time for them. There are also trips to walk on the Solheimajokull glacier in this area, but you need to provide half a day for it and contact the operators.
Vatnjokul ice cap
Similar question. This one's about half the size of Belgium. There are winter trips onto it, but the regularly daily schedule of trips only becomes established in the summer. Again you'd have to allow half a day for it, contact an operator, and note that the access point is about 20km E of Jokulsarlon, where there is accommodation.
Given you are going in mid-March, which is ice-cave hunting season in the Skaftafell area, guide necessary, you might think about contacting an operator and booking a trip to do that, as it is popular and the trips book out.
late lunch in Hofn
It really isn't worth driving there and back from J, it's just another 150km in the car. Hofn itself is of fairly little interest, except for the view, but you'll have had a lot of view, weather permitting, all the way from Vik to J.
Vestmannaeyjar?
It is interesting, but you really need to give yourself a proper day for it. Getting there on the ferry, etc, takes time, and then it is large enough to take some time to walk around and see the views. But one of its attractions is the seabirds, but you are too early for those, so you won't have the full experience of it.
whale watching (seems like it's not the right time of year?).
Trips do operate all year, sea-state permitting. They do get cancelled much more often in the off-season because of rough sea-state, and the sightings don't seem to be as exciting either, but these things do change. Maybe some particularly interesting group of animals has recently got into the habit of visiting the area, if so you'll find out by looking at the operators websites. A couple of the operators keep a trip diary on their websites, so you can go back and look at what kind of an experience they had last March, if they were operating then.
Blue Lagoon?
It's your money, you can make the call on that.
