There is a good deal of confusion (as well as frustration) when it comes to the idea of "Impromptu" vs. "Improvisation" . . . seems most think running about pockets filled with an arsenal of gadgets and novelties now constitutes "Impromptu" . . . it's a Boy Scout thing in which one is "prepared" to jump through hoops upon demand. While this can be fun it can likewise set you up when it comes to doing lots of free work and the expectation of "friends" for you do give them a free show when you're invited to parties at their home, etc.
Improvisation on the other hand, is where one learns how to be a real magician. You come armed with two things and two things only; your physical skills of manipulation and your mind -- that memory core that should have enough material in it so as to allow you to become creative and resourceful. For an example, stealing off a strand of long hair from a young lady so as to replace the IT you're used to having on hand (for many a century, this was IT), learning to look at available objects in ways that give you a variation to playing cards such as photos, business cards, even the backs off match books. . . and so the list goes on.
Stevens Magic has a video series on Bar Magic and Impromptu Magic that you would do well in investing in. They are hosted by some of the top close-up guys out there and more importantly, those that work as Magic Bartenders or barroom hustler styled performers. At the same time, just investing into manuscripts that will help you see things in a unique way and likewise, allow you to understand how to exploit circumstance such as doing the Cups & Balls (or even a Shell Game) using clear water glasses or simply coffee cups and sugar cubes. Ring & Chord routines are a natural in that you can most always find a shoe lace and finger ring that can be borrowed (and if you happen to have a Thumb Tip on hand, you can do some fantastical "Ring Flight" type bits as well as bill into just about anything).
WHEN IT COMES TO MENTALISM the list of available things one can deliver at the improvisational level is staggering IF you have studied to become an actual Mentalist vs. someone dependent of trickery/gadgets. There are several Serial Number I.D. methods that are available to you as well as ways to nail down a person's astrological sign, to know which hand has a particular item and for that matter, who within a group has the item in question and so the list goes on. . . If you are even half-way savvy in working with Pendulums and other "effects" that rely on ideomotor response you can start your own church on the spot . . . as Banachek points out in his famed book on Psychophysiological Thought Reading as well as Disc 3 of the PSI SERIES DVD collection, there is very little you can't do when it comes to Muscle Reading skills. My observation being that Muscle Reading and Cold Reading will pretty much give you a decent career in and of themselves; toss in Billet Reading skills and you can become a God within the Psychic world.
Billet Work being the next best thing when it comes to improvisational mentalism in that you can borrow business cards and writing instruments most of the time these days let alone napkins and coasters set about a bar or dinning facility . . . we have plenty when it comes to what can be used as a Billet, the rest stems from your personal studies of technique and routine.
As a Mentalist I've taken great pride to have a short list of bits I can move into based on a given environment and set of circumstances. It's not that difficult to have an easy 30-60 minute set that's been rehearsed and is ready to go as a whole or in segments. This can include any number of bending type demonstrations for those that feel they need to torment innocent keys & flatware.
THE BOTTOM LINE is simple; become as proficients as you can with Slight-of-Hand and in so doing, challenge yourself to manipulate a wide variety of objects and items and to likewise obfuscate certain "naughtiness" such as we do, without being obvious. Someone studied in slights and perhaps some rudimentary pick-pocketing skills can easily deliver 20+ minutes of fun. You just need to look at your various environments and think things through prior to being placed in such a position (or imposition, as it often is). Think things through, write it out and practice it by challenging yourself over and over and over again . . . Be Prepared but not by having ten decks of cards in your pockets and a bracer of rubber bands running up your arm.