As we know there are crewed mission plans to go to Mars and people has been in the Moon already. Has there ever been crewed 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.
Yes, there have been and are multiple crewed mission concepts to celestial bodies other than the Moon and Mars, but they aren't covered by the media that much.
As stated in another answer, a Venus flyby in the Apollo spacecraft and Saturn V's third stage was a concept for the 1970s, but never realized. It was part of the Apollo Applications program to which SkyLab and the Apollo Soyuz Test Project belonged.
Around the turn of the millenium the concept came up of a manned mission to Callisto and establishing an outpost there, called HOPE, as stated in another answer. Callisto is the outermost Galilean moon of Jupiter and thus the only one where humans can remain permanently (with current technology) because it is far enough from Jupiter and thus its dangerous radiation. Callisto is a bit smaller than Mercury and an ice moon which may harbor life under its surface where a subsurface ocean is suspected to exist.
Current concepts for missions to celestial bodies other than the Moon and Mars include the High Altitude Venus Operational Concept HAVOC which is a plan to enter the atmosphere of Venus where the spacecraft would become an airship and the crew would float about 33 miles above Venus' surface on the equator. It would float in a speed that a day on Venus would be about 24 hours for the crew. At 33 miles altitude the atmospheric pressure of Venus is like Earth's surface pressure and the temperatures are around 50 degrees Celsius like on Earth in a hot summer. Astronauts wouldn't even need a pressurized suit if outside.
Then there are plans for a manned mission to an unspecified near-Earth asteroid with the Orion spacecraft that is planned to return astronauts to the Moon in 2024. However these plans are rather on hold because the lunar missions have priority now. Formerly it was planned that an asteroid would be trapped into lunar orbit and a crew would be sent to it. Note however that near-Earth asteroids are tiny and have little mass and gravity. You can't call that a "landing" but rather a tightening to them since you'd be in microgravity.
Well, and while there are no concrete concepts, many people think steps beyond: manned missions to Mercury (either its night side or a crater of eternal darkness), missions to main belt objects, to Europa, to Ganymede and to Saturn's moon Titan.