Despite what you may read about the forum, unconfirmed transactions work plenty well enough for this sort of thing.
What if you send a 0-fee TX that doesn't exceed the priority threshold, so it will sit unconfirmed for days, but then you are able to double spend it back to another address of yours with a fee when you get back home?
Of course there are risks.
But how often would this happen?
1. If you do a double spend in a shop or a bar, you will never shop there again or drink a beer there again
2. We are not talking about values of 1000nds of $ here, but a normal Bill at a bar or shop
3. If that's you reasoning as a shop owner, you should also not accept Credit Cards, because the person could always do a charge back later or the Credit doesn't have any backing
4. The normal user isn't even able to do a double spend and it will get less and less the more Bitcoin goes mainstream, but everyone can do a CC charge back or pay without having enough backing on his account.
In addition there is always the "risk" that your transaction actually confirms.