Trip to Denver - Any suggestions?

Hello everyone,
I would like to get suggestions on my trip to Colorado for the 10th.
I would be staying in Downtown, Denver for two nights and then driving up north to Estes Park and staying there for 3 nights.
Please give me suggestions in places that are worth while and fun for a 20 year-old.
Any car rentals?
Famous Hikes?
Fun activities?
I'm pretty open-minded, so anything will be a nice experience!

Thank you for your help!!!