Beautiful looking bike!
I would change very little at all. But, since you're looking for ideas... here are some changes I had made on my 86 touring bike - which I had just purchased last summer. And, I will probably be making some more changes myself... eventually, although nothing further is really needed.
http://www.flickr.com/photos/strongl...7605576341958/
1. Definitely mount some Fenders. Even silver colored SKS P45 plastic fenders would look great, they're strong, and they're only around $35 with all the hardware (think I paid $32 at the local REI outdoor equipment store for a set last summer, now they're up to $40).
http://www.rei.com/product/684330
2. Bar-end shifters would be a nice convenience touch. I like the Rivendell "Silver" NON-indexing sets which are a bit pricey, but they will give you very easy light action shifting. And, without the limitations of the index feature you are never locked into a specific freewheel or cassette cog count.
http://www.rivbike.com/products/list...product=17-089 - Keep an eye out for them on eBay for a good deal.
3. Those look like Dia-Compe cantilevers. Mine are a model 981 and they were designed to allow use of either 27" or 700c wheels. Here are a couple photos which show the vertical "adjustment slot" concealed beneath the cover... keep your fingers crossed, perhaps your's will switch over easily too!
http://www.flickr.com/photos/strongl...7606108523956/
4. New cantilever brake pads are definitely a good investment and will make a dramatic difference in braking!
5. New cables and housings will make everything operate perfectly!
6. Ride it for a bit and consider just changing the chainrings. I suspect those may be something like 30-45- 50? and there may be a more comfortable middle chainring size which you would use more... or at least would be more familiar shifting across.
7. You may be able to adjust the rear hub a bit to use a "compact" 6-speed freewheel without modifying the rear spacing of the frame at all. My bike is spaced at 122 mm. It is set up with the original 5-speed freewheel but it has a wide spacer on the left side (and LOTS of extra room on that side), so the wheel was definitely built up with the potential for moving the spacer to the right and then using a 6-speed freewheel. You would have to re-dish the wheel slightly to center it in the frame if you do this, but that is easier than changing the entire spacing of the frame. Depending on your cog and chainring selections, this could be all you might ever need! The narrower the rear spacing and the less "Dish" required [as on a mega-speed modern cassette], so the stronger the rear wheel. So, typical touring bikes back in the day were often built with fewer rear cogs than were actually available even at the time a bike was built.
8. Personally, I'm torn between leaving my original 27" rims or changing to 700c rims (which I actually happen to have already). The biggest advantage would be the easy availability of a greater variety of tires - especially better quality ones.
9. I have randonneur bars too and the spacing at the brake lever height is definitely narrow. And, on the SR bars the width between the brake levers was narrower than most others bars of that style. But, they are also flared out quite a lot at the bottom. This makes them very comfortable to ride in the drops without ever hitting your forearms on the tops of the bars. I always have my bars adjusted pretty high - about saddle height - so I do ride in the drops quite a bit... and very comfortably. Before changing the bar, perhaps consider raising the stem a bit and try it out for a while.
Again, that's really a wonderful bike you have there. Seems like very little would really "need" replacing beyond the normal wear items.
Enjoy!
Bob