Tandem bike, but with no partner. Still too subtle? Try putting a sign on the other seat that says, “this seat open.”
Any costume at all, but stare far too long at any woman (or man) that walks by.
Get a piece of poster board and go as “Facebook.” Display your relationship status as SINGLE. Still too subtle? Have a bunch of wall postings that no one has liked, except other clearly single friends.
Any costume where your cat is dressed up as part of a themed outfit.
Kissing booth. Still too subtle? Have a sign offering to pay people to let you kiss them.