It's only about 4 or 5 kilometers between Moulay Idriss and Volubilis, so when the weather is nice, it's very pleasant to walk from one site to another.
Moulay Idriss is of course most famous for being the resting place of the founder of the Moroccan kingdom. He also founded the city of Fez and united the berber tribes to create a vaste empire.
It's also a very nice little village where you can easily stroll through the streets of the medina. There is a little square in the center with cafés and small snacks. You arrive at the foot of the rocky hill on which the city is built and then wind your way up through narrow streets until you reach the top and get a great view of the whole valley. You can't visit the Moulay's sanctuary if you are not a muslim but you can get a good view of the building from the top.
Here is a much better shot that I stole from Kazuhisa Otsubo.
Combined with the trip to Volubilis, it makes for a great day out from Meknes. If you don't have your own car, you can either take the collective taxis from Meknes or hire out a driver for the day.