On the surface of the planet, without terraforming, it would be virtually impossible to construct a habitat where humans could live. If one was going to do such a thing, they would have to find a way to reflect an extraordinarily high amount of heat, somehow power the base, create fuel, and protect against the acidic compounds of Venus's atmosphere. Nothing conceivable could be done to last for any length of time in that environment, let alone build a habitat there. In addition, you would have to build weak points to get in and out, which would be difficult.
There are a few other possibilities, however. Below the surface of Venus, a habitat could be constructed which would work. Also, if one could build high enough supports, the atmosphere about 50 km up is actually quite similar to Earth's, one could live there, given the support structure. Other than that, I can't imagine any way to live there without some sort of terraforming.
The minimum terraforming required would be to cool the planet down. This actually would be relatively easy, essentially all one would need to do is build a giant tin foil shade, or something similar, in orbit and keep it there. If the atmosphere cooled sufficiently, it would start to thin out, eventually making it much more manageable. It would be somewhat easier to get Mars to be more of a habitable planet, but it could still be done to get Venus there, someday.