There are many types of visual puns. The most obvious are literal puns. For example, you could make a candy dish that looks like a piece of candy, or a serving platter that looks like a record, also sometimes called a platter. Another idea is to create a cereal bowl that is decorated to look like a cereal box on the outside. Think about what you want to make, and then think about how you can be very literal in your interpretation of that item.
Turn your washable ceramic items into a commentary on our throw-away culture. Mold coffee cups that look like crushed paper mugs, or plates in the shape of newspaper sheets. These are clever items that draw attention to the need for less disposable items without lecturing. Instead, they'll make the viewer smile, which is more likely to make the point hit home.
There is well-known optical illusion in which two faces in profile, when looked at just right, form a vase in the negative space. One idea for a visual pun with ceramics is to reproduce this stunt by producing ceramic faces that, when put together just right, produce an anti-vase. The pun being, of course, a ceramic artist producing a vase from thin air.
Take a look at the ceramics around you and think about how you could flip them upside down. Put your signature on the top of the plate and the design on the bottom, for instance. Or design a cup so that it seems as though the person using it is drinking from its bottom rather than the top. The effect is an Alice in Wonderland style that will cause people to look, and think, twice.