A habitable colony world on a moon orbiting a gas giant in the goldilocks zone for their parent star. The main character should also visit there at night, when the view of that monster in the sky is at its most spectacular.
Also cool would be a colony that mimics some of the ideas for colonizing Venus. Basically it would be a cloud city that floats miles above the surface of a Venusian hothouse planet.
It might also be interesting to visit a habitable world orbiting a red dwarf. Red dwarfs are by far the most common stars in our galaxy, so they might also be the most common stars to host life on their orbiting planets, assuming life exists in the universe beyond Earth. That sort of planet could be interesting because life there would look very different from Earth...
Sunlight fuels life on Earth by powering photosynthesis, a process that converts solar radiation and carbon into sugars. Our sun's color, temperature, and distance from Earth have coaxed photosynthetic plants to absorb most wavelengths of light except for infrared and green, which these plants instead strongly reflect.
But most stars in the nearby universe aren't like the sun
About 80 percent of the Milky Way's stars are dim red dwarfs. As a result, astrobiologists have suggested that photosynthetic plants on worlds orbiting lone red dwarfs could take on hues of red, blue, yellow, purple, or even grayish-black to best absorb the starlight.
"Planets have to be about five times closer [than Earth is to the sun] to support photosynthesis," he said. "Red dwarf stars are more active, and their solar flares will hit the surface of the planet more often. That's a problem for life."
As a result, O'Malley-James thinks land-based alien plants may evolve natural sunscreens to protect themselves. If the plants are in water, they may temporarily sink when they sense bursts of radiation to protect their vulnerable photosynthetic molecules.
"That plants might move—to sink or shrink under a rock to evade a solar flare—is a really cool idea," said Jane Greaves of the University of St. Andrews, an astrobiologist who is O'Malley-James' doctoral supervisor.
Alien Trees Would Bloom Black on Worlds with Double Stars