Thanks, all, for the suggestions.
I decided against flowers, thinking that might require the overhead of her dealing with a vase.
I didn't know whether there'd be food... thinking sticky fingers on piano keys might be suboptimal, so opted against chocolate or cookies.
I wound up giving her a copy of Charles Cooke's little book "Playing the Piano for Pleasure."
As for the gathering, I had a lovely time.
She lives 160 miles from me, and I overcaffeinated a bit for the drive. (It must have been that, right? And not nervousness at playing for more than three people in what was now the fourth occasion for me since the early 1960s. Right?)
There were seven of us. All but two were people I'd met at the Tonebase "intensive" in L.A. last May.
Our hostess has a 6-foot Yamaha grand.
We each played a bit, then broke for refreshments. During the refreshments, various of us played a bit more in the background.
Much as I'm grateful for my electronic Avant Grand, the action on her piano was like a sports car in contrast to my little pickup truck. Much faster. Much greater dynamic range than what I have, but my uncalibrated touch wasn't able to adjust well enough to take advantage of that. Much louder at full stick than what I'm accustomed to with my headphones at home. Really a nice change of pace.
54 degrees and foggy when I left home that morning at 9:30. 84 degrees and sunny at her house midday. 50 degrees and foggy when I got back home a little before 8PM.