The local dollar store sells pairs of "fuzzy socks" for $1. Except they are striped and in neon colors like yellow and green, pink and green, etc. Nobody in their right mind would want to wear them because they're scratchy due to the little strip of "silver" on the ends of the fringed yarn. They're also a bit too small for most adults, and a bit too large for most kids. Perfect useless present.
Chinese finger traps.
A bag of paper shreds (you know, the paper stuff people put in packages as "dressing"? Just a bag of that, and nothing else)
You mentioned a pet rock. Just go out, pick up a rock, paint a face on it, and "sell" it as a solar-powered, off-the-grid, ultra-green pet that never needs feeding, watering, walking, etc. (Although for a giggle, I've seen one packaged with a tiny folded-newspaper 'litter box' and care instructions)
I've seen "Fake chest hair"
small containers of pet treats, especially if you know nobody owns a cat/dog (or fish food)
I've seen plastic gloves that were blown up, and then drawn on to create smiley faces, turkeys, reindeer, etc.
A small sheet of bubble wrap
Toilet paper cozy (you know, the things you put over rolls of TP to "hide" them. My grandmother made truly hideous ones in the 70s.)
corn removal pads
Snowman poop (bag of marshmallows)