How long did you guys practice together?
How many of the songs you were learning did you play?
Did they seems surprised or happy with what you did learn?
You happy with how the new pickups worked and did you get the sounds your were hoping for? Things can sound different playing with a full band.
Did you have moments that you felt like you were connecting with them.
What did you think of the drummer?
Do you have more songs to learn before the next practice?
What do you think of the singer?
Which guitar did you take to play with them?
Do you know how many songs they know that makes up their set list?
I'm not much of a letter writer but I'm full of questions...