As we know there are manned mission plans to go to Mars and people has been in the Moon already. Has there ever been manned mission plans by any Space Agency, to go to a solar system object other than the Moon and Mars, such as Ceres or an asteroid?
There wasn't much point in doing it, as it would have been several months of flight for a very brief approach to Venus, in which the crew couldn't do much that a robot couldn't do much more cheaply and safely.
Before cancelation of Constellation program in 2009 one of planned missions was manned asteroid landing and return. The same Orion spacecraft and Altair lander would be used as for the Moon.
Also manned landing on Phobos qualifies, I think. It should be easier than landing on Mars surface (with return, of course). There is no atmospheric entry problem. Also Phobos-landing craft does not need a lot of propellant/several stages for return.
I am not sure if this reaches the level you considered of "plans" but NASA had the HOPE (Human Outer Planets Exploration) studies, which involved flying a beautiful nuclear-powered ship to Callisto.