I've actually started going in the opposite direction - swarms of encounters that are substantially easier than normal. Last session we finished 8 fights in 4 hours; that time also included a lot of roleplaying, dungeon exploration, and skill challenges. That's with a party of 5 6th level PCs. The secret? 4th and 5th level monsters. It was still challenging, as the party is time-limited and so they were trying to kill the enemies while taking the minimum damage possible. It was fun for me, since combat never started grinding.