The are 2 different ways across 2 unique modes of transportation to help you travel from Denver to Thornton. Take public transit from Denver to Thornton, it's your best chance!
The earlier you can book your trip to Thornton, the better. There is usually no need to book these sort of trips from Denver to Thornton in advance. Rideshares should be available on demand all hours of the day. Public transportation prices do not fluctuate like flights. Instead, they're either flat rate or priced by distance traveled. If possible, you can book your tickets via an app or alternatively at the station upon arrival.