Venus

Surface Temperature: 735K
Atmosphere:  Oxygen/nitrogen mixture at roughly 21:78 mixture ratio; clouds of sulfuric acid droplets
Atmospheric Pressure at Surface:  96 Bar (1300 PSI)
The surface is cloud covered; with little or no solar energy.

Settlement Concept

"Although the surface of Venus is an extremely hostile environment, at about 50 kilometers above the surface the atmosphere of Venus is the most earthlike environment (other than Earth itself) in the solar system. It is proposed here that in the near term, human exploration of Venus could take place from aerostat vehicles in the atmosphere, and that in the long term, permanent settlements could be made in the form of cities designed to float at about fifty kilometer altitude in the atmosphere of Venus."

Landis, Geoffrey A., Colonization of Venus,
NASA John Glenn Research Center, (Feb 2003)

The Landis paper referenced above is the primary source for information regarding the Venus alternative.  Key aspects of this paper are summarized below.

At 50 KM above the surface:
Temperature: 0-50C (habitable)
Atmospheric Pressure: 1 Bar (equivalent to that on Earth)
Gravity:  0.9 Earth norm
Radiation:  Protection from atmosphere roughly equivalent to that provided by Earths atmosphere.
Though above cloud level, there is high wind velocity at this level of the atmosphere.
Building Materials:  Venus offers natural resources that could be mined to support construction.

Scientific Challenges

0. Would have to develop materials resistant to corrosion by sulfuric acid.  This would apply to all equipment exposed to the environment.
0. Developing a floating habitat that can provide a stable environment within the high velocity winds.
0. Highly capable exploratory robotics that are corrosion resistant and can withstand the extremely hot temparature of the surface, above 450C.  These robotics would have to be controlled by fully immersive virtual reality technologies.