As long as we are dreaming big dreams about colonies on the moon and Saturn's moon Titan, how about making a cloud city into reality? Not on Earth, of course, but floating high among the cloud tops of the outer gas giants such as Saturn, Uranus and Neptune, where there is no solid ground to stand on.
The idea is more feasible than you might think. One surprise is that gravity turns out to be a non-issue -- even though these planets have many times more mass than the Earth, the fact that they are gas giants makes them much less dense than our planet, reducing the force of gravity at the "surface". For instance, at the altitude where the air pressure is at one atmosphere, the gravitational force is a benign 0.9 G on Saturn and Uranus, and 1.1 G on Neptune. (This is unfortunately not true of Jupiter, where the gravitational force in the cloud tops is around 2.4 G.)
Right now, nobody is planning any balloon missions to the gas giants. But I wonder how hard it would be to construct a large "cloud base", suitable for permanent habitation. Obviously you would start with unmanned balloon probes, which would find each other and link up to create a platform. One of the early "pioneer" balloons would have to contain a massive power source, preferably a sizeable nuclear reactor -- with the distances involved, solar power is not an option.
At some point the balloon platform could only grow by producing its own materials, as "importing" them from outside is ridiculously expensive. The atmospheres of the outer planets are abundant in hydrocarbons, mostly in the form of methane, so synthetic materials could be produced in conjunction with the energy from the nuclear reactor. From this perspective, Saturn might be more attractive, as its atmosphere contains significant amounts of nitrogen (in ammonia), oxygen (in water vapor), and phosphate (in phosphine), allowing the creation of more complicated plastics. Robotic labs in the balloon platform would separate out the required elements from atmospheric gases, react them into plastics, form the plastics into structural elements, and construct the station -- all of which would have to be automated processes.
As the mass of the station grows, the automated construction process would have to produce new balloons. And before the first humans arrived, the automated construction process would have to produce pressurized living spaces, including life support systems. Humans could travel to the station in a special space vehicle that converted into a blimp after entering the atmosphere; the blimp could maneuver into the station and dock with it for personnel transfer, like the Goodyear Blimp. Getting away would be a little trickier. It is possible to extract rocket fuel from atmospheric gases, but the rocket would have to be enormous to achieve escape velocity, which is 2-3 times that of Earth (the lower density does not help you here). Sending humans away from Earth is a challenge even for the largest contemporary rockets, much less rockets made out of plastic by an automated robot.
Still -- as fanciful as all this sounds -- it has previously been made as a serious proposal, in a paper entitled "Helium-3 Mining Aerostats in the Atmosphere of Uranus", by Jeffrey van Cleve. Outer gas giants have enormous stores of Helium-3, which is a potential fuel for fusion energy. In the distant future, Helium-3 might be a highly valuable substance, making a permanent mining colony on a gas giant feasible.