Here is an example of a workout I am doing:
Pyramid Hill Intervals. Warm up well and then do the following intervals at your CP6 power (zone 5b) or VO2 Max level. My CP6 is 380-400 watts.
Do 1-2 minute efforts at lower cadence 60-80 rpms, and 3-4 minutes efforts at 80+ rpms: 1 minute at CP6, 2 minutes at CP6, 3 minutes at CP6, 4 minutes at CP6, 3 minutes at CP6, 2 minutes at CP6, 1 minutes at CP6, (total = 16 minutes at CP6) Alternate between seated and standing climbing. Recover for time equal to interval (for example, 2 minutes on, 2 minutes recover).
These efforts may seem somewhat easy at first, but towards the end they start to get difficult. It's also best to use a power tap or some other unit that measures power because HR will lag behind. These require a day or two of "recovery" type rides or a complete day off before doing another such workout. I am doing my intervals on either Wednesdays and Saturdays, or Tuesdays and Fridays.
If you are looking for a challenging workout give these a try. They are best done on a slight incline (6% gradient) and on a road without any stop signs or lights. We've all got our favorite climb, right.