Since the most common and largest living spaces in the Solar system (and if conditions are similar elsewhere, then in the galaxy) are oceans hidden under the crusts of ice moons, then the most common post human adaptation would be to become fully aquatic beings living under the ice in these oceans.
The first condition to solve (and actually the easiest) is to invoke the "square cube" law and create large, blubbery whale like body forms for post humans. Tweaking the hydrodynamic form so the body is more spherical or "football" shaped than long and cylindrical improves the ratio of surface area to volume in order to minimize the flow of internal heat to the cold, exterior oceans.
The new, post human you
Manipulating organs like arms and hands would have to be withdrawn into the body when not in use in order to minimize heat loss. Similarly, other protrusions such as antenna or sonar or chemo sensors would also have to be internalized somehow, only extending as much as necessary and only when necessary. Energy management would have to be a critical resource for the post humans.
Since there will be little or no light, bio luminescence will have to be incorporated in the genetic design. In tune with the basic survival rule of minimal surface area, the skin of the creature will have to be the organ which provides the light, rather than some sort of "lantern" like deep sea anglerfish.
Bio luminescent body
The last critical item would need to be finding ways to provide the necessary energy for post humans to survive and thrive in these environments. Since the oceans are dark, photosynthetic plants and the conventional food chain would not be possible. However, the reason these moons have oceans in the first place is because they have internal heat sources, generally though gravitational "kneading" between the huge gas giant primaries and the resonant orbits of smaller moons in the system. This energy source will endure for geological ages, so the civilization is not totally dependent on artificial energy sources. Biologically speaking, the geothermal actions in the planetary cores provides energy to create the equivalent of "black smokers" injecting chemicals into the oceans. Post humans living in these environments will be using chemosyntheses in order to gain the energy to live, or more likely create entirely artificial genetically engineered food chains and ecosystems to enhance their environments.
Hydrothermal vent. This would be the oasis where life would radiate from
As for intelligence and communications, the post humans would adapt the tools of whales and dolphins, especially the organs used to create and focus sound waves for communication, sonar and other tasks (post humans might enjoy hunting by using the sonar organs to herd and stun prey).
So post humans, seeking to colonize the largest and most common living volumes available, will engineer themselves into beings similar to whales, and with adaptations to meet the particular challenges of living in cold, dark oceans.