I saw something on Pinterest for a nice basket of scented soap, lotion, and candles. The card said something along the lines of I hope you enjoy your shower as much as I did.
I thought that was cute, but I think I am getting manis/pedis for my mom and sisters as a thank you. I don't think they would appreciate the shower supplies as much as I would.
I voted for a bottle of champagne but my perspective is skewed by the fact that I really wish I could drink a bottle of champagne. I like the GC for a nice dinner idea. I would rather have a nice date night with the hubs than have a spa day by myself.
I have a shower this weekend too! I'm planning on getting the hostess a gift card for date night with her husband. Your idea for a spa gift card sounds great too!