It seems likely that the show will come up with a new thing to deal with the summer hiatus, and I was thinking on a few ways this might happen. (I know they didn't do the summer break for season one, but there was the writer's strike and that pretty much messed everything up.) The two I like the best are ...
1. Sheldon realizes that he has feelings for Penny, but figures that absence will make them go away and so takes a sabbatical and moves back to Texas for the summer break.
The problem with this is that it might be too soon for something that drastic and it would be either completely buried upon the fall return, or explored, and again, might be too soon. So I much prefer this other option:
2. Penny gets a part in a small, horror-movie that will film in Transylvania. It would explain her being gone for that short of a time, wouldn't be something big enough that would change her lifestyle, but would still show *some* growth in her career. And finally we could see (a) Leonard NOT hung up about it, proving that he is over Penny once and for all, and (b) but Sheldon is somewhat hung up about it, showing that those feelings are beginning to grow.
So what are your potential hiatus ideas?
ETA: Note, there are brief spoilers in a few of the comments.