Neptune Colony: Major Fusion Fuel Exporter
Neptune is a gas giant planet. Some of that gas could be used in aneutronic fusion reactors. How does one mine gas from a giant planet? One method described in the video is to create orbital rings that resemble a chainswa. Buckets are lowered down into the thick atmosphere and brought back up. Along the way automation in the buckets can refine the gas. It's something you might expect an eight year old to come up with but that process is technically feasible.
Using stellasers mentioned in the colonizing the sun video solar sail ships can be pushed toward the outer planets. With a huge amount of power from fusion reactors around Neptune lasers can fire beams which would push solar sail ships back toward the inner planets.
The atmospheric winds of Neptune are also a potential source of power. Constant strong winds could run huge generators similar to jet engines.
In some ways colonizing Neptune seems similar to colonizing Venus. Instead of floating in something like a blimp a habitat could be suspended from a tether in the atmosphere of Neptune. That makes less sense to me though compared to Venus. With Venus you could wear a protective suit and go from the habitat to the surface for a limited amount of time. There is no solid surface for Neptune to work on. It's a lot of gas pressing down on lower layers and the pressure causes the lower gases to transition to a liquid state. Below that is a solid core but there's no protective scuba suit that could handle the intense pressure.