Believe it or not, you can fall in holes that DON'T have pitfall seeds in them! You need the updated version of Animal Crossing New Leaf to do this.
1. Dig a hole right in front of any rock.
2. Sit on the same rock
3. Turn to the hole while sitting on the rock
4. Jump off the rock, above the hole.
Once you jump in, you'll be stuck in it like it's a pitfall seed! Enjoy!
(Also I would've put this in the Cheat forums of Animal Crossing New Leaf but I didn't know how. I have made a post before as a question)