Various avenues in garden design exist. If you live in the desert and are water wise, use a drier garden scheme. But if your landscape gets plenty of hydration, consider large water features. Arid or tropical in feel, bright and loud or subtle and quiet, creative ideas are endless.
Fashion together porous rocks (use either real rock or create your own from cement) in a suitable size and design to create a mountain. Leaving pockets for soil, plant this creation with various species of mosses, cactus, alpines and sedum. For the more adventurous, install a working waterfall in one part of the mountain. A small kiddy pool at the base of the rock holds water and provides additional garden space for aquatic plants. An additional kick to this idea is to use scale animals (mountain goats, puma) tucked into various areas of the mountain.
Aztec Ruins Flowerbed
Using cement try creating authentic looking Aztec ruins. Molds are available online for purchase. Burying the edges of the pieces makes the arrangement realistic. Try encouraging moss to grow on the surface of the decorations. An oasis (pond) can be added and "gold" coins scattered throughout the bottom giving the idea of lost Aztec gold. The Aztecs had magnificent gardens, so planting herbs and other medicinal plants throughout the area would work well. Using vines and arbors will veil the area, giving it a secluded and forgotten feel.
Old West Train Flowerbed
For a fun garden experience, set up a working (weatherproof) train station and track. Creating hills and mountains with mounded soil adds height to the illusion. A low-growing groundcover and careful placement of stones and "trees" will prevent soil erosion. Partially bury rocks in the hills. Make tunnels through the hills by using PVC pipe sections and installing the train track through them. Paint the pipes black or a soil-matching color to help them blend in better.
Adobe houses are scattered throughout the west. Craft miniatures from native soil or purchase the supplies from garden centers or online. Using dwarf, slow-growing plants and the correct scale buildings and animals will add realism to this garden idea. Irish moss, dwarf conifers, miniature sedums and small cacti are just a few species that work well for this theme.