fun things to do with 5 and 10 year old in Iceland

Hello,
We will be doing a 10 day Ring Road trip with 5 and 10 year old kids in a couple of weeks. We'd love to hear any advice about fun things to do with kids that age. They love interesting but not too strenuous activities (5 year old, especially). I wonder if there are any operators allowing 5yo to horse-riding; what are the best thermal pools; where are fun hikes and where it is best to see the puffins. These are just some ideas - and we are open to any others.
Thank you !