Without trees to generate oxygen, humans would have to retrieve it from scuba tanks and the minty breath of local supermen. Keep the air alive with today's Groupon to Denver Botanic Gardens. Choose between the following options:

• For $14, you get two general-admission tickets (up to a $25 value).
• For $25, you get four general-admission tickets (up to a $50 value).

The green-thumbed staff at Denver Botanic Gardens cultivates exotic flora in numerous gardens with more than 700 species of native plants from South Africa, Japan, and other blossom-draped regions of the world. Patrons stroll on walkways through gardens thriving with fragrant flowers such as roses and lilacs, as well as desert foliage, including cacti and vine-covered cowboys. The Boettcher Memorial Tropical Conservatory houses thousands of exotic species such as banana and coffee plants, and Marnie’s Pavilion parades orchids against a backdrop of tumbling waterfalls in a balmy building. Volunteer tour guides lead educational treks through the plant-bedecked facilities, dispensing wisdom and teaching guests how to milk dust from a tumbleweed.

Rotating art exhibits explore a diverse mix of topics and media, such as the outdoor sculpture collection Native Roots | Modern Form: Plants, Peoples and the Art of Allan Houser, which runs until Sunday, November 13. After viewing galleries and etching replica paintings onto stray flower petals, visitors can head to the Gardens' café to sip coffee or nosh on organic, locally sourced fare.