Many rivers are more or less the same. I guess you would have more than enough to do here wether you arrive in early summer or late summer.
If you are planing on running some creeks on the east coast (by many considered the place to go paddling) early summer is better. Also a few (2 I think) good rivers close to Akureyri are dependent on snow melt and rain.
I guess the most important thing for you is wether there will be paddlers free to go with you, there are not so many of them.
Around Reykjavík (less than 2 hours driving) there are several good class 3-4 rivers which run all year. Then there is one beutiful river in Skagafjörður which is good all summer. One river (Hólmsá) close to Kirkjubæjarklaustur is class 3-4 runs all summer, also good. So yes there are many things to do. And I didn´t even mention the waterfalls.
Have fun in Iceland. Too bad I can't help you more, but if you are ever in Switcherland...
jon
