Does anyone knows if there are buses to drops us on the starting point again?
Yes. It is possible but it is tricky doing it all in one day, it is better if you have an overnight somewhere away from your car. You have to change bus at either Hvolsvollur or Seljalandsfoss. You could also leave your car at Hvolsvollur and have a bus trip at each end. Here you can find timetables for the Thorsmork buses - NB 2 operators need to check both their timetables, which are on separate pages. Here. http://www.nat.is/travelguideeng/bus_schedules_iceland.htm You will also find buses from Skogar, again 2 operators, look at the Reykjavik-Hofn timetables, again 2 operators, 2 pages. In addition there are a further 2 buses per day between Hvolsvollur to Skogar (doesn't call Seljalandsfoss) run by Straeto whose timetable is here http://www.straeto.is/media/sumartimi/Leid-51-sumar2015.pdf I leave you to work out which direction is the best way to arrange it. Remember that the walk from Skogar to Thorsmork is very long, 10 hours is a typical walking time (and that for a fit hiker), so if you are aiming to catch the 1800 bus from Thorsmork (which enables you to get back to Skogar same day) it would be wise to set off from Skogar at 0600, so you aren't worrying too much about whether you'll make it in time. You may find it more enjoyable to book an overnight in Thorsmork so you aren't worrying about catching the bus after the walk. You could also get the 2000 late bus out of Thorsmork, stay in Hvolsvollur, go get your car in Skogar next day, and other similar arrangements.
is it possible to rent a kayak on the lagoon?
No
Can we camp there?
It's illegal to do so. With a car, there's no excuse.
Any nice circular trekking to do around?
You can walk to some other glacial lagoons in the area called Fjallsarlon and Breidarlon. Google will suggest some walking routes, but maybe not exactly circular. A lot of the best walks in the area aren't circular.
