Hill intervals! I went to an area I "discovered" a couple weeks ago with the workout plan of hill intervals. This area has a lot of hills to try and ride and others that are very ride-able along with some very fun single track and some almost too steep decents. I did three main climbs and really worked myself over with them - they were steep enough that I was fully geared down and out of the saddle to get up them. To make it harder, they were covered in rocks from pea gravel size to "baby heads" , so you had to continually adjust your line while also keeping a very circular pedal stroke to keep from losing traction and/or momentum.
By the time I finished my final climb, I was pure jelly throughout my legs and even arms. Sure sign of a great workout!