Sean Raf's answer is a good one, but let's just, for arguments sake, say that you DO want to show something. I have not seen an abundance of good answers on this thread...parts of good answers...but nothing really usable.
Here are some solutions:
Consider effects that are self working (this has been said, but no recommendation on where to find the stuff, so...), you can find some great stuff in "Card College Light"...and I haven't read "Card College Lighter", but I would expect the same. The self working effects shouldn't LOOK self working, they should just be moveless.
No matter what the shape of the deck, as you will get decks from dollar stores with holes in them already, shaved edges...apprently shaped like carrots - you should be able to do something. I can think of MANY things that would fit the "dealing only for the effect to work" genre, and would allow you to use ANY deck.
Also, like mentioned, you don't have to use a deck...borrow a coin, keep rubber bands on your wrist...this not only promotes versatility, but allows you to be prepared at all times. You can find impromptu effects from sources like, Gregory Wilson's "On the Spot" and I think Dan Harlan has a set of videos on impromptu stuff as well, just to name a few.
Moreover, the motto is "Always be prepared...or loaded, if you like?" - I keep an effect in my pocket that plays like an invisible deck - I can't remember the name of it, and am too lazy to go look it up for you, but it was just reviewed in December's Mag Magazine as an effect you do from your wallet with "Invisible" cards. Anyhow, there are other effects like this...that you can keep in your wallet. Perhaps a good time to carry packet tricks - or create a trick that is meant to be JUST for when you are put on the spot and don't have cards.
We have all heard it, but a poor worker blames his tools...so you have two choices - make sure you are always prepared to do something - ALWAYS - by (one) keeping something on you at all times that makes it easy to do...or (two) being prepared to do something with what you have around you.
Then again...you can always do what Sean said..."Not right now", as you are off duty, and are not required to do "command performances".
Hope this helps.