While a gradual climb on a road bike is certainly the most controlled environment to complete intervals on (or a trainer), I race mountain bikes and sometimes I need to mix in some skill-work within my workout. As long as a little flexibility with power output is ok, I've found doing hill repeats on my mountain bike quite rewarding!

Here’s one of my go-to workouts:

5x5 minutes hill repeats at Zone 5 (race pace) with 5 minutes rest

Here are a couple of notes to make sure you execute this workout:

1. Locate a relatively consistent 4-6 minute climb up a road, dirt road, double track, or even some singletrack for this workout. The critical thing with this is that the uphill interval is paired with a descent down a fun trail. 
2. The aim is to go at slightly above race pace (zone 5) up the climb and at the top drop into your singletrack descent straight away or after a very short rest. Being able to handle downhill trails while gassed is part of cross country or enduro racing, so dropping in quickly is an important part of this exercise.
3. Total rest, including descent, should be around 5 minutes, any extra time left in the rest portion can be done at the bottom of the descent.
4. Don't forget to take your AltRed 1 hour before starting the workout!

These can be done in a 5x5 minutes fashion or broken up into two sets like 3x3 minutes, rest for an additional couple minutes, then 3x3minutes again. These are an awesome way to get the hard work done while still being on the mtb, practicing off-road skills, with a fun reward at the top of each interval! Give this one a try!