There's lots of tours you can book online in Reykjavik without going through Regent. Haven't really looked, but they're probably cheaper also.
Everyone who has time should definitely do the Golden Circle tour, it's probably the most interesting tour we've ever taken--however it is an all day tour, so you need one full day to do it. The other really fun thing to do in Reykjavik is the Blue Lagoon. It's kind of a hot spring type thing but the bottom is a silica mud that you can put all over your body and it makes your skin really, really soft. It's supposedly very therapeutic. The only problem with the Blue Lagoon is that you may freeze your tushie off when you get out. Reykjavik is not a warm place--so bring jackets, etc. Just doing a tour around Reykjavik is pretty interesting also--there is a hop on hop off bus you can take. You can find Iceland tours on the Icelandair website or by just searching Reykjavik tours.
Editing to add: Marcie, I agree--I wish Regent would get their act together and let us know what the deal is on the hotel. I'd probably rather stay at the Hilton myself. Also, weren't we going to get a private van to pick the six of us up at the airport? And there's still hope that Regent will pony up for the transportation I think.