A chill settled in over Denver’s Civic Center Park over the weekend, and it had nothing to do with the seasonably warm temperatures.

An easygoing vibe embraced the concert stage at the inaugural Outside Festival like a climber clinging to a rock wall. The weekend was like one giant musical bear hug.

Officials estimate about 8,000 came out for Saturday’s lineup, headlined by the soulful Thundercat’s six-string bass seductions. About 10,000 more ventured out Sunday to swoon and sway to the whistling Andrew Bird’s virtuosic violin and the intoxicating, churchlike harmonies of Seattle’s Fleet Foxes.

The eclectic 10-band lineup had a distinct international flair, with the intercontinental Say She She bringing the disco glam and The Heavy Heavy leading their infectious clapalongs all the way from Brighton (not ours – England’s). Denver’s own Los Mocochetes are deeply rooted in Mexican funk, and Selasee is a reggae artist from Ghana who lives mostly in Boulder.

The only thing they all have in common is their highly uncommon niceness, which was a pervading theme that extended throughout this first massive, mellow gathering whose entire reason for being is to promote Colorado and the lucrative outdoor industry. But there were some recurring musical themes: Funky rhythms, face-melting instrumental jams and harmonies that were bursting at the seams. Feeling groovy.

This particular confluence of festivalgoers produced what had to be just about the most polite festival crowd anywhere, from the affable musicians to the discombobulatingly courteous security team to the well-mannered, diverse, all-ages crowd – which makes sense given that a gathering of those who are kind to the outdoors would also be inclined to be kind to each other.

Say She She, a Brooklyn-based combo fronted by three fashionable, forceful sirens straight out of an “Austin Powers” film, demanded that those gathered turn and dance with a stranger – and they dutifully abided. Their mantra? “Joyful elation and spiritual elevation.” Most impressive, they hung out after their set for hours chatting up fans and checking out the other bands – while still sporting their glam high-heeled boots. (Check out “Forget Me Not,” an homage to New York City’s Guerilla Girls – an anonymous artist collective that wears gorilla masks and supports women’s rights movements.)

Lettuce, a classically trained, six-piece Boston instrumental groove band inspired by Denver’s own (mostly) Earth, Wind and Fire, said or sang little until the very end: First, they announced they would be playing a surprise drop-in set later that night at Cervantes’ Masterpiece Ballroom. Then, this sendoff, apropos of the whole weekend: “Unity is all we need. Tell each other, ‘You are right,’ even when they are wrong.”

Stephen “Thundercat” Bruner brought the thunder with his megawatt smile, falsetto vocals and signature six-string, hollow-body bass guitar. (Check out the Pink Floyd riff “Lost in Space.”)

The most unexpected non sequitur of the weekend? Thundercat asking the Denver crowd: “Is anybody here part of the Mile High Club?” followed by the telling rejoinder: “There aren’t too many of us.”

On Sunday, The Heavy Heavy came on like a shot of joyful adrenaline. They’re led by led Will Turner and Georgie Fuller, a woman who gives off a definite Exene Cervenka vibe – if Exene were a happy person. Fuller is all-in on the party, which with this band is an amalgam of psychedelia, blues, acid rock, sunshine pop and, oddly – the soundtrack to a 1960s Western. (Check out “Go Down River” – the most Outside Festival song an Outside Festival song can possibly be.)

Andrew Bird’s foreboding set opened with the closest thing to a political statement all weekend: The powerful ballad “Bloodless (for Now)” speaks of a coming uncivil war with lines like, “Certain actors are reaping power and wealth from divisiveness.” “Atomized” furthers Joan Didion’s contention that individuals can be broken down into isolated parts. Other songs imagining impending peril included “Inside Problems,” “Sisyphus” and “Manifest.”

More than anything, though, Sunday night was for the fleet of heart. As in, the widely adored and (again) uncommonly civil Fleet Foxes, which brought a multitude of strings, horns and honey harmonies to the final set of the weekend. Opening with “Sun It Rises” as the sun, it set, impossibly polite frontman Robin Pecknold held sway over the swaying thousands with vulnerable, plaintive campfire confessionals like “Grown Ocean,” which dreams of a day without darkness or cynicism.

Other songs included “Phoenix,” a Big Red Machine song Pecknold helped out on with Anais Mitchell; “Mykonos;” “Blue Spotted Tail;” and the ultimate Fleet Foxes gateway tune, “White Winter Hymnal.” Pecknold sent them all home dizzy with “Helplessness Blues,” a song of distant love with vulnerable lyrics like, “I don’t need to be kind to the armies of night that would do such injustice to you.”

Lastly, a word of advice to festival organizers and city-event planners: Now that downtown concerts (often not free) are now commonly staged in front of the glorious City and County Building, it’s time to convert those two ornate streetlight poles that frame the stage into collapsible, removable items that don’t create unnecessary sightline problems in the otherwise open-air atmosphere of Civic Center Park.