How are you defining "bustling and full of activity"?

Gold (from 1858) and above all silver (from 1878). The state has plenty of other resources, but it was precious metals that kickstarted growth. Cattle ranchers moved in from the 1870s followed by sheep-farmers and by the 1900s arable farmers. But Colorado remained ahead of the curve in urban population. And only half of it's mountain.

Geography and resources. Colorado had a silver rush which not only created mountain towns, but helped to firmly establish Denver as the local hub for miners. So in its early years a lot of money came into the city. The hills just west of town are shot through with clay they used to make the bricks that built the city, which might not sound like much, but having a source of durable building material close by was important.
Probably most importanly in terms of resources is that Colorado is a watershed for 8 US states. In the western US, water is power. We also have tellerium, uranium, gold, diamonds, ranchland, and oil/gas resources. In short a lot of money has been pulled out of the ground here.

Because of its centrality it also became a transit hub. Denver Municipal Airport, later to become Stapleton international was opened in 1929. It had been a central hub for various airlines over the years until it was replaced by DIA in 1995. Denver is the jump off point for all of the state's mountain based tourism, which we have a lot of.

In the recent past, the 90's tech boom was huge in Denver and the surrounding suburbs. A largely socially liberal, fiscally conservative city at the edge of some of the most breathtaking natural environments in the country turned out to be a big draw.