I take it you mean Ayr when you mention the Bard. Actually Alloway, just outside Ayr, is his birthplace and there is a visitor centre and Burns cottage there with a museum containing original manuscripts.
Good luck canoeing the Spey and I hope the anglers are hospitable as you pass. There is also a well-established rafting/canoeing enterprise on the River Tay which you might like to explore also and it takes in places such as Kenmore, Pitlochry, Dunkeld down to Perth. As far as castles, there are an abundance but Dee-side might be your best bet with Braemar, Balmoral, Corgarff, Crathes, Craigievar, Drum and others all along its length - and you can also do some canoeing too!
As to driving, the roads are not too hectic and the locals fairly forgiving. Just remember to keep to the left!