Beyond Selidor

I'm so excited for you going - I hope you will love it as much as we did!

Vik and Jokulsarlon sound great, would have been my recommendation, too. I was also thinking about one geothermal area but both we've been to on this side of the Island (there is another one on the Northern side) are not really accessible with bus. I think this one may be: http://besthiking.net/hveragerdi-hot-spring-river-trail-hiking-iceland/ if you add a bit for getting from the town/village to the beginning of the trail. It's pretty amazing and absolute recommendation (take your bathsuits with you, it's worth it!)

If you are thinking about going to the North, Arnarstapi would be a great stop - the basalt coast is absolutely amazing. The hike is super easy (more of a walk) but the views are just stunning.

I would also recommend to make some (guided) hike to one of the glaciers. We haven't planned properly ahead here and ended up driving a mud road and then hiking at some random location - I think it would be even greater if better organized (and I've heard good things about actual glacier walks, but as said, we haven't planned this part properly and then it was too late ...).

(P.S. Some of my impression are here http://pax-athena.livejournal.com/264157.html - but please don't feel forced to look/comment. I would not be pointing it out if I did not think that it could be useful.)

Definitely stop at Seljalandsfoss - and go the 200m inlands to take a look at the second waterfall there; this one was perhaps the most impressive of all I've seen.