Less well known are the plans for manned Venus missions. Venus is the closest planet to Earth, but that's about all it has going for it. Temperatures on the surface are hot enough to melt lead, and the pressure would crush a man instantly. The atmosphere is composed mostly of carbon dioxide and sulfuric acid; definitely not a life-friendly environment. Despite this, there were planned for manned flybys as far back as the 1970s. The American plan would have used Apollo hardware, while the Soviets would have used an uprated N1 (the N1F) with a heavily modified Soyuz as the crew module. Neither plan came to fruition (due to budgets cuts / changing priorities in the American case, and the failure of the N1 for the Soviets). Instead; we (mostly the Soviets) sent dozens of unmanned probes; mapping the surface from orbit, investigating the atmosphere, and parachuting in landers that lasted for minutes at a time. Despite the inhospitability of the planet, you'll occasionally hear about plans for missions to Venus; some of them even include deploying blimps in the upper atmosphere (there are a some layers of the Venusian upper atmosphere where the temperatures are survivable).
Far less has been written about manned missions to Mercury. That planet is farther away from Earth than Venus or Mars. Also, it's a lifeless, barren rock; with no atmosphere to speak of, the sunward side is roasted, while the night side experiences cryogenic temperatures for weeks at a time. Mercury's magnetic field is horribly weak compared to Earth's, meaning that the surface is exposed to obscene amounts of radiation (this is also why Mercury can't retain an atmosphere). Far less is known about Mercury than other nearby planets; the first probe (Mariner 10) did flybys in the 1970s. The MESSENGER orbiter expanded mankind's knowledge of Mercury greatly, but compared to the Moon, Mars, or Venus, there's still many unanswered questions.
While a manned mission to Mercury would be highly difficult, it would certainly be possible with future technology. In fact, it could probably be done by 2050, assuming sufficient funding and will were available. While that's unlikely to happen (indeed, there are far better targets for exploration), it's still an interesting problem to look at.