There are quite a few of these "see the highlands in one day!" tours out of Edinburgh. There is so much bus time. The best thing to do is explore close to Edinburgh and resolve to go back. :)
If you really want a touch of the countryside, You could do one of the shorter tours to the Borders and see one of the Abbeys. Or i if you want to say you've walked in the hills of Scotland, take the train from Edinburgh to Dunkeld/Birnam. There are some very nice not stressful walks to the Hermitage and along the River Tay. And if you go over the bridge to Dunkeld you can check out the cathedral, the restored little houses, and have an ale and lunch at The Taybank. There's a sweet little Beatrix Potter Garden in Birnam.
Or another great day trip is to Stirling to see the castle. It's amazingly historic. I am pretty sure that there is a bus out to Bannockburn--there is a big anniversary this year and the referendum on Independence is not coincidentally being held close to the anniversary.
You're going to be in Scotland very close to the Independence vote.