An Abundance of Katherines
This is probably Green’s funniest and lightest book.
Colin Singleton is an ex-child prodigy who is addicted to anagramming and dating girls called Katherine. When the 19th of that ilk dumps him after graduating high school his best friend takes him on a road trip to stop obsessing over the break up.
I read it for the first time in 2009 and I enjoyed it then too (though I was apparently going through a phase of not actually reviewing and rating books at that point).
Recommended if you want a funny and sweet, sort of romantic, teen novel.