"Minimal gear" may be the question here. I've been caught in similar situations and now I have one essential setup for most solo appearances: my keyboard, keyboard stand/bench, 2 small powered speakers, some lights. The keyboard acts a PA rig since it has mic input and internal mixer/eq so I don't have an external mixer. I can do my normal show with this. Setup is 15 minutes flat with a sound check and takedown is the same.

This rig is also my excuse for not doing many things such as allowing guest singers or other nonsense. I have one mic and one cable that is only long enough to reach from my keyboard stand mic adapter to the keyboard (plus a spare just like it). No CD players (I have break-music midi files).

This eliminates surprises and anything that I am not prepared to accomodate should be blamed on poor planning on the client's part. I do have a full PA and can provide it at competitive rates.

Some people will try anything to get you to do more, a lot more, than what you've been hired/paid to do. I'd go up to the comfort level of obliging your clients with the true minimal equipment you have brought and after that I'd begin judicious use of the word "no". If that's not good enough for them, they are only out to rip you off anyway.