It depends on how complicated you want to make things and how good you want the final sound to be.
The easiest thing to do would be to get a guitar synth. Hook it up so that you can use a foot pedal to adjust the volume between your guitar and the synth and fade in the synth sounds as you play.
If you add a sampler via midi, you can easily add real strings or any other sounds you want too.

If you want to get more involved, go the MIDI route. That way you can have a computer play all the keyboard parts in time with the band. All you need to do is have your drummer get used to playing to a click and program in all the keyboard parts ahead of time. At the same time you can have the computer control the patches on your multi-effects pedal and the lighting if you have the right equipment.
The major drawback though is you have to learn how to use recording software and spend a lot of time pre-programming everything.