We are spending 5 nights in Iceland before embarking - the first 3 nights in Reykjavik and then renting a car and driving west to Snaefellsnes Peninsula for 2 nights.

Here are a few companies I've come across offering day excursions from Reykjavik.

I've read good things about this one - we are taking their tour to the South Coast & Thorsmork:
Iceland Excursions

Nature Explorer
We met one of the owners/super jeep drivers of this company at a temple in Bhutan! We are signing up for his Golden Circle super jeep tour.

the biggest operator is:
Reykjavik Excursions


for private tours:
KGB Tours

There are lots more companies if you want some links.