For me, the best MUni Weekend trails are scenic, long enough to be out there for most of the day, include challenges for riders of all levels, and enough strenuous stuff to come home tired, hungry, and ready for a beer.
Friday: Mr. Toad’s Wild Ride, Lake Tahoe
Mr. Toad’s starts high (about 2300 meters) and starts with a good, strenuous, Totally Doable climb (to 2700 meters) through alpine forest, with occasional views of gorgeous Lake Tahoe. After a rest at the peak, you’re treated to a long descent (about 11 km), which starts out with a mix of rocky, rooty, super-technical stuff, including some long staircases, and then finishes up with a fast, swoopy singletrack section for anyone who brought a geared uni.

Saturday: Porcupine Rim, Moab
Arguably the most spectacular MUni scenery anywhere, Porcupine Rim is known for shots of Krazy Karl and other nuts jumping on cliffside rocks, but you don’t need to be insane to enjoy this trail; there are great, complex technical bits all the way down the 1000 meter descent to the river. Every time I go back I ride something I didn’t manage to ride the last time; it’s a perfect place to advance your MUni skills. And now you can shuttle all the way up, so you don’t have to do the 2-mile sloggy climb to get to the Rim.

Sunday: CBC trail, North Shore Vancouver
I think this was one of two or three trails we linked together that day; I don’t remember them all. In any case, the North Shore is in many ways the birthplace of modern MUni (thanks, Kris) and the trail toys are really well built, and amazing for skill building and generally getting your adrenaline levels up.

Amusingly, two of these trails have been featured on California MUni Weekends, but none of them are actually in California. There are some great CA trails, too, but I love these three.