The bus service in Iceland is excellent and travels mostly on route 1 ...

which takes you near Godafoss, Lake Myvatan and will get you to Akureyri.

To get to Grimsey your best bet is by aircraft which can be rather pricey there is a ferry but I am unsure of where it docks at. The bus service does run to all the ferrys

from Akureyri I would rent a 4 wheel drive if possible because the side of the river you want to go up to see Dettifoss is on the West. Red falls is several KM up the road and a short hike and the other falls is a short hike up the river. The area like Maine is quite a few Km after that. I did meet a few people that road their bikes and hiked out there. It can be done and the buses will let you bring bicycles. (There are good roads on the east side of the river but the view from that side is the difference between seeing Niagra falls from the American side and Canadian side.)

When you go on the Golden Circle tour keep in mind that to the Icelanders this is their great place of government where it all began. To a naturalist these are the monolifts above the Atlantic Ocean and if from the lower parking lot you hike over the hill you will see a beautiful waterfall which can actually be hiked to the top and just before the church there are a bunch of crevises which people use as a wishing well. This was once a place where they sacrificed virgins. An incredible site to see and a digital camera 512 mb will be easily filled on just the Golden Circle tour. I would advise that you have at least two large capacity cards and plenty of batteries.

If you find you can't get to the North to the south of Iceland is an island called Vestmannaeyjar which is where a major volcanic eruption happened about a decade ago. The cliffs are very interesting. (I also likes whale bay and snaefellsjokull but these are for people who want to spend months there. Iceland has ruins of a 10th century fort in the five fingers but it is difficult terrain and the roads are often closed)

If you take the bus the whole way around the island on the southern side you will get close to their national forest ,Glacier bay, vatnajokull glacier, Myrdalsjokul glacier, Vik and skogafoss which are all close enough to the road to hike to.

When you get to Reyk you need to pick up the guide called Around Iceland which has all the phone numbers etc... in it. These should be available at the main bus station.

I did a ton of camping in Iceland I would pull my truck off the road and I set my tent up in various places. I never had any problems however this I found out is frowned upon. They want you to staty in campgrounds and hostels.

One of the few things that bothered me about Iceland is high end restaurents do serve whale meat, and they also eat puffin, reindeer and horse. So do be careful about what you eat if this would bother you.

Remeber to look around I went to Vik 4 times before I discovered this awesome little beach with incredible cliffs cut by the waves. There were also caves full of birds all over the island, natural bridges and just things that blew your mind because you happend to hike down a hill at low tide.

One of the nice things is that you are allowed to go on peoples property to see things like waterfalls,caves, lava tubes and craters (and you are not charged a fee) unlike the US. These are to be shared with the people not harbored by the rich.