I wasn't sure that it would necessarily count towards the Thief Achievement (it does), but I started to reverse pick pocket "victims" in Skyrim.
My favorite thing to place in their pocket was a random food. If they were nice, I might give them an apple or a sweet roll (mmm).
If they were evil buggers (I'm looking at you Imperials), I would give them a carrot or a piece of cabbage and pretend it was fermented kimchi.

Muahahaha!

What other goofy things have you done in your game?

I like to provoke chickens into violence with Frenzy and then make a bet with myself on which one will win the fight!

I messed around with Ulfric Stormcloaks corpse and dumped it on the dining table in front of what was his throne, letting the jarls I had deposed during the Civil War eat while staring at his naked corpse.

That reminds me...

http://i693.photobucket.com/albums/vv299/Le_Fried/2011-12-24_00012.jpg

http://www.youtube.com/watch?v=4v-JuDaFWKw

I couldn't resist the urge of making this video...

I like to go tubing on the rivers, where I start floating with the current way upstream and end up at the mouth of the river at the sea.

You should throw random items in with you and race them down stream!

In your book, dear OP, is the goofiest thing I did probably to have joined the Imperials! Muahahaha!!!

I use shouts like Kyne's Peace in places like Dragonsreach to mess up the position of all objects. Now its a mess in there.

Blasphemy! LOL