The primitive shapes (Point, Segment, Disc, Rectangle, and so on) are controlled by the Shape parameter, which is an integer parameter with 10 possible values. So you could just plug a Random Value node into the Shape port, set the mean to Box, and the variance to 4 or 5. Note that if the random value goes out of range, you get a point.

If you don’t want points or segments (or some other shape), you could just use a Randomized Value node to drive a Select Case node: