Loving the fact that the OP felt the need to make it clear they meant the country not the shop when asking.
This place is superb: https://www.thingvellir.is/ In general it’s very expensive. We took a 3 litre box of wine in our suitcase. If you’re self catering it’s worth an extra luggage allowance and taking some food with you - pasta, bread, cold meat and cheese in a small cool bag etc. check for any import restrictions
Should also have mentioned the public baths are great. People go there to chill out and socialise. If you’re brave you can use the plunge pools from the very cold to the very warm.