Good idea. There are three candidates for human exploration that come to mind as being pretty easy because they have very little atmosphere -- the Moon, Mars, and Mercury. Human landings on Pluto might be possible, depending on how the surface looks when the New Horizons probe gets there. Ceres, Vesta, and Pallas are also possibilities. The larger moons of Jupiter and Saturn appear to have atmospheres and nasty ones at that. The difficulty of landing and taking off has that added to it.
http://www.freerepublic.com/focus/f-chat/1294934/posts?page=12#12
http://www.freerepublic.com/focus/f-chat/1447339/posts?page=4#4
O lord, I wasn't referring to actually landing on the gas giants, I was referring to getting near them period! We can't even put probes on the gas giants without them liquefying in minutes.
Planets with solid spinning cores, like the Earth, create a magnetosphere that acts as a shield against radiation. This is cool. Where these field lines are strongest particles in the field are ionized, or given an electric charge. When we look at the Aurora Borealis the lights we see are these ionized particles.
HOWEVER, when you have something the size of Jupiter with a spinning solid core, the ionization is much more severe and very deadly. These ions could kill you by frying cells in your brain. You would never see them or feel them, you would just drop dead. The gas giants have an invisible field of these ions whipping around. These things are not just deadly to humans, they're dangerous to probes and we've had to toughen the probes we've sent out there to account for this. They can fry circuits and cause all kinds hardware failures.
I believe Mercury and Venus are out of the question. Mercury is too close to the massively radioactive sun and Venues has a greenhouse gas effect that melted a Soviet probe in about two minutes. It's pretty much Mars, Pluto, and the asteroids and moons in between in terms of actually visiting and staying alive.