Other than a couple of the top-tier Disney World resorts, Marriott's Mountainside in Park City is my favorite resort on earth. Usually when my family goes to a resort, we spend little time there, going off property during the day and sleeping at the resort at night.
Mountainside is different. They have awesome activities for everyone in the family, every day, all year long. We've visited this resort in the summer and in mid-October, and the staff was at the top of their game during both visits. It's not unusual for us to spend most of our stay on the resort property, enjoying the hot tubs, pool, gym, and scheduled activities.
We usually buy groceries in town and have relaxing meals in our unit.
The units are nicely furnished, comfortable, and with all the amenities of home. I do wish they provided a mid-week cleaning for timeshare units, but they don't. However, they are happy to deliver clean towels or extra plates with just a phone call to the front desk.
My kids learned to make sushi, paracord bracelets, and duct tape wallets. One evening we enjoyed free brats we roasted over their fire pits and listened to an excellent blues band. My daughter participated in an art class held off property (Paint Mixer on Main Street) and had a great time.
I can't recommend this resort highly enough and Park City is a wonderful destination, year-round.