What Eric said, and you could also have a look at some of the software VST hosts. A few years ago, I used a host called Live Professor for my keyboard gig rig, running on a laptop.
I had two MIDI controller keyboards (weighted piano controller and an old Hammond XB2 with drawbars) connected to the laptop via a MIDI interface, and a tiny audio interface with one output to my amp, and the other output going to the PA. All I had to do upon arriving at the venue was to power up the laptop and load my gig setup, and it would be ready by the time I had the keyboards set up and plugged in. I had all the sounds I needed ready to go - piano, Hammond, strings, Rhodes, Wurli and various synths. I had each VST instrument on a different MIDI channel, and was able to switch between them quickly by changing MIDI channel on either of the keyboards. That way, I could have piano + Hammond, piano + strings, Wurli + synth etc.
You could have a bunch of your favourite VST instruments ready to go on a laptop, and you'd just have to load the setup at the start of each session.