One of the most beautiful gift a public can offer to an artist is to show, through their applause, how much they were touched.
This is the gift the public of La Journée de l'Audace Genève gave me a few days ago after my show-talk.
Individual discussions afterwards also touched me greatly, and I know those conversations will last for long.

Another kind of acknowledgement, more technical, is when the sound engineer thank you for making his job easy.
This is also one of the gifts I received during the event: as he was overwhelmed with technical requests from all around, knowing that he only needed to enable an XLR input on the stage so that I could share some music, reduced his stress level immediately.
"Be a solution rather than a problem" is a philosophy that always pays off.