Hmmmm... Some interesting advice given and some very good ideas, though IMHO why not take a 'bite' out of a coin?
1) Borrow a coin from person. Have them mark it if you want.
2) "You know how many germs are on this thing?" as you wipe it on your shirt sleeve.
3) Do a french drop and 'pop it' into your mouth and swallow.
4) Open your mouth and show coin is gone and say, "Sorry, I had a real small lunch and I was still hungry. But dont worry, it will pass."
5) Reproduce the coin from wherever you want and hand it back.
1) If you have a nickle in your pocket and as you get it out (dont let them see it) ask to borrow a dime.
2) Switch (your favorit method) the dime for the nickle and bring it to your mouth.
3) Click the coin on your teeth and then hand them the nickle... hehehehe you bit the coin in half!
I know it sounds corny but at the same time, you do not have to make an excuse because you do not have your gimmick. You have shown them something and perhaps it will come across stronger than the coin bite for them since it is completly different than what they expected and not gimmicked.
And remeber, if you use gimmicked cards, learn some things that dont take a gimmick. I have a special rising card deck that I like, yet I know how to do a rising card without it that is completly on the spot and can use their cards.
I hope that these ideas will help.