I know you said shuttle isn't an option but they have a good service for 45 bucks you can get from dia to breck. Which will be much cheaper than a taxi. here's a link
also I have hitchhiked in colorado a good amount you can get to the mountains pretty easy that way. find some kids looking your age at the airport or on your flight see where they're going if they can't take you up the mountains. or closer towards the mountains then hang out at a gas station put on a big smile and see if yo ucan't get a ride from someone headed your way.
I'd offer my help but I won't be back in colorado for another three weeks.
there is ride share a carpool site that helps you find someone who is going your way and can give you a ride. I did this once in high school and i just gave the guy some money for gas, and I got lucky and we both had buds on us so we got baked the whole way up. here's a link
you can also try craigslist and make a post and see if anyone can give you a ride from dia to summit county.
Taxi is always an option.
Uhm those are about all i can think of. good luck may the wind be at your back and the sun on your face on your way to colorful colorado.
"kinda miss my kid, sold it for my vibes ticket and some dose last year hahah. but i still shred on the hand drums like it's my job." -DFAC