Certain objects may also be set to have the avatar sit on them with a simple left-click, (by setting "Left-Click Sit" on the edit window.) Such objects will have a chair icon next to it).
Objects that react upon being sat on, (such as teleporters,) can have their pie menu text changed with the LSL function llSetSitText. Often, the avatar never actually appears to sit on the object in question.
An avatar can not be sat on by default though there are ways to simulate this with scripted objects.