I'd recommend saving your money and exploring by bike on your own.
Our guide from Green Discovery walked us over to a shop that was renting bikes and we borrowed from there so the bike quality wasn't any better than we could have got for much cheaper. The lunch that was provided wasn't very good, and the food that our guide bought himself from the cafe at the Blue Lagoon looked a lot better. Although it was nice to have a guide show us the way, I think a map would have done the trick just as well, and we could have hired a guide at the cave entrance (which I'd recommend).
In short, it was a pleasant day, good scenery, nice cycling (though be prepared for a lot of bouncing), but not worth the added cost of going with the tour company. Much better that you do it yourselves.
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