I like to keep working when I'm away from my big machine so I'm looking for a minimal program I can use in the office.

I want something that will support 1 MIDI track, 1 audio track and 1 VSTi - any recommendations?

Thanks - rev

Perhaps Cakewalk Music Creator 6 would be the way to go.

Although I've since abandoned it as a potential candidate for being my primary DAW, one of the things I liked about Studio One is that it worked well on a laptop. 

I took it with me on holiday one year, to a remote island where I had very limited internet access, no musical hardware except a tiny Akai keyboard, no books, no software or sample libraries except what would fit inside the laptop's hard drive. It was literally an "island setup". 

Forced to concentrate on nothing more than a MIDI track and software synthesizers, I was able to focus entirely on melody and harmony. I felt as though I'd returned to the essential roots of music by stripping away all the external distractions. It was refreshing. That little Akai keyboard now has a permanent home in my laptop case so I can whip it out anywhere. And Studio One is still on the laptop.

And there's no reason not to use your full version of Sonar/ As long as you don't build a large project any computer can handle it.
I keep a copy of Sonar on my office computer just to sometimes play with stuff when bored. Only issue you find is how much Sonar hates most on board sound cards. My office computer has a Sound Blaster so uses ASIO. But this is the one good application for using asio4all.  
Now by office we are talking about your home office right?
 
When I installed it I did a custom install and did not bother loading a lot of the heavy duty VST's that I knew would not work. Like Guitar rig.

You might want to look at Reaper.