I wouldn't change any of the original parts, your bike has some really really nice components on it. You could probably dig around for some brake hood replacements. Suntour Superbe was the top of Suntour's heap, so in a period correct sense there is not really any upgrading. You could upgrade to some nice brake pads.

What I would do is overhaul everything, fresh grease in the hubs, bottom bracket, headset and even the pedals. New brake and shift cables, clean up the chain and take a look at the tires and tubes. I would treat the frame with some framesaver and polish and wax the paint. Don't forget to lubricate all of the pivot points on the components with something like tri-flow.

What you add depends on how you want to use your bike. You could add a rack and pannier bags for commuting or camping. The possibilities are endless really, just take time to think about your goals and plans for this bike.

Thats a really awesome bike and well worth the few extra steps.