I suggest cooking the butternut squash in the crockpot, if nothing else is destined for that particular cooking vessel.
Here's a recipe for roasted butternut squash with brown sugar. Butternut squash soup is also really tasty (and you could cook that the day before and just warm it up) if you don't mind adding in a soup course.
As for other vegetable sides, I'm always a fan of green beans with slivered almonds, and roasted garlic-y brussel sprouts. You could also do a green salad as a side, no cooking involved with that!