Thanks guys, the reason we want to stay in Vik on saturday night too is because it'd be getting quite dark straight after the ice caving tour and in the event of inclement weather I'd much rather be driving to Vik than Reyk I think (plus the Hotel has a nice out door hot tub we could make use of on Saturday night!). Going to hire a 4WD or AWD from Blue Car Rental, they put Snow studded tyres on for free this time of year.
Try
www.rentalcars.com , they worked out the cheapest for us and tell you which local company it is before you book (Ours was Greenmotion). From memory it's illegal to not have winter tyres in Iceland so all car rentals should have them when you go. Tell you the fuel policy too (take full to full!).
If there is only 2 of you, don't rule out the Suzuki Jimny. It may be compact but it's a perfectly capable 4WD and if you put the back seats down it has enough cargo space, certainly not an option if there are more than 2 of you though! It'll probably be the cheapest 4WD on offer for hire to boot.
Other general tips:
Most petrol pumps are card operated (ie not paying at kiosk) so make sure you're clear to use whatever card you will be. As usual the Halifax Clarity credit card is the best option for overseas spending.
Main supermarkets are Bonus and Kronan. Bonus is the main budget one (think Aldi/Lidl) and Kronan is your Asda/Morrisons equivalent. Groceries are on the expensive side as a lot of the veg is either greenhouse grown or imported and meat the same. We didn't find eating out too expensive but we avoided the touristy places (you can spot these by the whale meat and rotten shark menu dishes). An average meal seemed to be circa £10-15.
Icelanders love junk food, you'll see lots of KFC/Dominos/Subways etc. Most larger petrol stations have a grill as well selling burgers and other food. Make sure and check out the Icelandic hot dogs (Pylsur). They are delicious and made with lamb, beef and pork. Make sure and ask for it "eina með öllu" which means the works (Ketchup, sweet mustard, remoulade, fried and raw onions). You can get these in most petrol station grills and also most towns have a Pylsur kiosk or drive-through.
Do - try and learn some basic Icelandic phrases such as hello, please and thank you. Whilst most Icelanders speak English, they are a bit like the French and appreciate a bit of effort. There were a lot of Americans when we went and they were being their typical brash selves and you could see the locals rolling their eyes. We had the basics down and made an effort and they were very friendly and helpful at providing good information about what to do, what not to do, where to go etc.