"Present it to him. Set the hook. Pull your rod up to 12 o'clock. Now, snag him, snag him!" There follows about 10 seconds of giggles from my ponytailed fly-fishing instructor, Ryan Schmidt, who up until this point has been the epitome of laid-back fisherman cool, standing waist-deep several yards away from me downstream. But not now. Ryan has transformed into an excited child as the large rainbow trout he's had his eye on for the past half-hour has decided to gobble my fly and disappear to the depths of Eagle Creek.

Seconds earlier, I had "presented it to him" – to use fly-fishing parlance – and watched as the tiny, floating contraption made of feathers and cotton drifted with the current over the trout's head.

Now I'm trying hard not to trip up on the underwater rock that I'm precariously standing on and go swimming with him. Eventually, I manage to pull him to the shallows, but, as Ryan leaps down to retrieve my catch, it's all over – the trout has seen what's coming, somehow flapped around enough so that the barbless hook slips out of his mouth, and he's gone.

I'm in Vail, Colorado, to see what this ski resort has to offer in summer. When the snow melts, the crowds disappear, leaving a tranquil playground for anyone interested in hiking, biking, rafting, kayaking, fishing – or just lazing around enjoying the sunshine and the crowd-free mountains. Ski resorts in summer seem a bit of a secret, and, unlike the trout I had planned to have for dinner, I'm hooked. Even though we're a stone's throw from Vail village, just off the highway, once we're standing knee-deep at the edge of the Eagle River, we could be miles from civilisation. It looks like a scene from A River Runs Through It. I may be no Brad Pitt when I'm learning to cast, but this place is stunning. And a man can pretend.

Ryan had explained that the trick to landing a fish is to look around at the flies and aphids landing on the surface of the water. The bugs change according to the surrounding vegetation, the time of day, the stretch of river, and even the weather, and you should choose a fly to match. It feels like an art form – one which I'm certainly not about to master in four hours, unfortunately.

The following morning, I am back in the water – this time standing at the edge of a ferocious-looking stretch of the Colorado River, with white water cascading off huge rocks scattered randomly and diverting the flow in all directions.

There are six of us signed up for a river-rafting trip today, and we carry the inflatable raft to the river where our guide, Travis, gives us a pep talk before launching the boat. We bounce off a rock and then bob over the gentle rapids until we hit the big stuff.

"Left side! Paddle!", Travis shouts. We've been taught to lean forward, arms straight, and pull the blade back through the water. It seems to work, as the bow of our boat turns to avoid a nasty-looking rock, and heads through the middle part of the river.

It's clearly not adrenalin-fuelled enough for Travis, though. As we approach a milder stretch of white water, he orders us to join him in the back of the raft and lean back. "Let your life-jackets support you in the water," he says.

I later find out that this little trick is supposed to go something like this: as we lean back, the raft peels away from the water and stands upright on its stern, while the six of us float on our backs, legs gripping the inside of the boat. Here's what actually happens: nobody places their confidence in their life-jacket, and so we all lean back but refuse to touch the water. Inevitably, the giant rubber raft flips over, leaving my five shipmates in the drink and me clinging for dear life underneath the boat. When Travis flips it upright again, I'm sitting comfortably inside while everyone else is grabbing for oars with one hand and trying to grab the boat with the other as we plunge through the rapids.

Later that afternoon, safely back on dry land, I join on a group hike up Vail mountain – usually pictured under a blanket of snow. Our guide is Alex Spaeth, a seasoned hiker, mountain biker, and skier. We haven't got far when Alex stops and asks: "How do you tell the difference between a brown bear and a grizzly?" No one knows. "If you're hiding from a bear up a tree, and the bear climbs up after you, that's a grizzly," he says. "If he shakes you out of the tree, he's a brown bear."

It would be a charming anecdote if he didn't then show us the branch of a bush stripped of serviceberries – a sure sign a bear had been here recently. Still, it makes us walk a little faster.

The mountains here are beautiful. Lodgepole pine, juniper and ash blanket the horizon, only interrupted by the ski runs which at this time of year are overgrown with grass. There's plenty to do in the evenings here "off-season" as well, and you rarely need to book a restaurant in the summer. At Terra Bistro on the banks of Gore Creek, I tuck into rocky mountain rainbow trout, with a sweet potato hash; at Toscanini, the Italian restaurant at Beaver Creek which sources most of its organic ingredients locally, I have house-made ravioli stuffed with roasted butternut squash. All of which, of course, makes getting soaking wet during the day rather more bearable.

Travel essentials

Staying there

* Tivoli Lodge, Vail (001 970 476 5615; tivolilodge.com) offers doubles from $149 (£100).

* The Osprey, Beaver Creek (001 970 754 7400; ospreyatbeaver creek.rockresorts.com) has doubles from $170 (£114).

Getting active

* Gore Creek Fly Fisherman (001 970 476 3296; gorecreekfly fisherman.com) offers one-day fly-fishing excursions on the Eagle River for $375 (£250).

* Timberline Tours (001 800 831 1414; timberlinetours.com) offers guided family-friendly rafting trips from $80 (£55), including transportation, equipment and lunch.

More information

* summer.vail.com beavercreek.com