Denver is a car-friendly city with many parking lots available throughout the downtown area. You can also use ride-share services such as Uber or Lyft or take the public transportation which includes buses and trains. The city offers a 1-day pass for $6.
If you have extra time, consider visiting the Denver Zoo or exploring the city's many art galleries. If you only have a few hours to spend, prioritize the attractions that interest you most. Make sure to bring comfortable walking shoes as many of the attractions require walking. Also, be prepared for the city's changing weather by bringing layers.