My idea is simple. Implement a challenge command. By typing "challenge (character)", you will be prompted to input the stats/skills that you are challenging them to. Perhaps something along the lines of "challenge ecks", followed by "strength, agility" for your mop bucket race!
The other player will then be prompted to the challenge, something like "you have been challenged by (character) in the following stats/skills", followed by the list, and you'll be prompted to either accept, or decline. if you accept, the prompt could even ask you how much you wanted to holdback by!
To stop people from spamming this command, I would recommend that it have the same cooldown as eating, closing doors, etc.