Iceland
Interesting day trip from Reykjavik, although I went for a long weekend just for the diving. You get to scuba dive along the volcanic cracks where the European and North American plates meet. Utterly fantastic visibility of 100m or so underwater. You get guided through and can hire all equipment. The more experience you have beforehand the better to enjoy it rather than fight a hired drysuit for the first time. If you show you know enough they may take you to the lower levels, which are up to 40m deep and totally enclosed - great fun, but not for everybody. Go in wintertime as it is quietest then.
Several dive operations in Reykjavik - I used these guys: www.dive.is/Diving_Iceland.php?page=Silfra
Hólmaslóð 2, 101 Reykjavík, Iceland
+(354) 663 2858
Google map: bit.ly/GV8OjN
Many activities in Iceland can only be done via coach trip from Reykjavik; if you go to the places where they occur you won't always be able to do them; the only exceptions are the popular places and/or popular things like horse riding.
Go to Tourist Information; there are several official/unofficial ones in "downtown" Reykjavik.
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