Tell them you can't right at that moment because your chi is too low cause you haven't eaten all day and that maybe they should buy you lunch so you can replenish. Atleast you might get a free meal out of it lol.
I really like this suggestion- sans the 'you should buy me lunch' bit but that's just not my personal character I guess...
The notion that you must accumulate energies (which are in turn depleted post effect) in order to perform such feats emphasizes - to an extent- that this is not a gimmicked illusion or a piece of 'trickery'; in terms of "What didn't I see?" or "What did I miss?". However, again, if this building up to grandeur isn't in your character's comfort zone, there are many other outs to consider:
I personally have not performed many gimmicked levitations and do not incorporate them into my day to day life for a simple reason: if asked on the street to do something with the wrong shoes on, I have to divert their attention to a different effect or "settle" for a different way of floating; taking into consideration my surrounding environment, lighting, crowd placement etc.
On one hand I can count the number of times that I have performed a Balducci only to : A) be asked to do it again or B) be told that they saw that one on youtube, you're just _____. In the case of A happening, following the Balducci with even something as similar but from a different angle such as King Rising was a moment of awe for those watching. After an apparent fall such as choice B however, following it up with say, FLY was quite extraordinary to the audience and I was left feeling much more confident.
Returning to your "Can you get Higher" dilemma though, I've combined levitations with IET with magnificent results explaining that you "don't float, you control the area around yourself. Controlling a smaller area is obviously much easier, here check this out" and going right into something equally amazing in its own right whilst still tying themes and tricks together.
I hope this has helped and feel free to pm me or catch me on AIM
Always,
David