If you only had eight hours in Iceland, what would you do?

There are tour bus options depending on your interest, time of day and available time.
the blue lagoon is really wonderful and unique.

It all depends on the arrival time. If it's during the day, I suggest joining a walking tour (private or free) with CityWalk in reykjavik . Our guide Marteinn was amazing. He showed us places to eat, interesting sites and shared intriguing tales about Iceland. Here is the link: https://citywalk.is/
CityWalk also provides pub crawls and a unique tour called Money Talks in the evening. 
Hope this info helps.

If the weather is decent you might also consider visiting Reykjavik Open Air Museum,  a little way outside central Reykjavik:
https://visitreykjavik.is/arbaer-open-air-museum

Assuming your '8 hours' is from ou'll need 1.5 hours each way to drive from and to the airport so you'll only have 5 hours to explore.
There's certainly more than enough to occupy you for 5+ hours in Reykjavik....and, frankly, it seems odd to visit a country and not visit its capital, which is a safe, friendly and very walkable place. So I suggest you drive to Reykjavik then walk around and enjoy the older part of town,  visiting some or all of its sites: the beautiful Hallgrímskirkja, the National Museum of Iceland, the 871?2 Settlement Museum, The Culture House - Museum, the The National Gallery of Iceland and more..... 
https://guidetoiceland.is/reykjavik-guide/top-10-things-to-do-in-reykjavik
https://visitreykjavik.is/reykjavik-hidden-gems
There are lots of places to eat in central Reykjavik but if you want to be  touristy, treat yourself to a hot dog from the same stand as Bill Clinton did: Baejarins Beztu Pylsur. It's on the corner of Tryggvagata.