There are so many different options, it is difficult to know where to begin ...
Take the A85 from Oban through Crianlarich towards Lochearnhead. (Kilchurn Castle suggested in a previous post is currently closed for conservation works, although the outside can be seen...) Think about the short detour along the A827 to Killin and the Falls of Dochart. Back on the A85 to Lochearnhead, take the A84 head towards Callendar (coffee break and explore.) Think about doing a detour on A821 to Loch Katrine and go for a 45 minute cruise on the loch. continue on A821 to Aberfoyle for lunch. There are a couple of easy walks you could do from here:
Continue on A81 and A 873 and consider a slight detour to Doune for Doune Castle and/or Dunblane Cathedral.
You can then pick up the M9 (busy) past Stirling for Edinburgh or alternatively take the A9 and A907 which takes you along the north side of the Firth of Forth through Clackmannan where you pick up A977 and then A985 for Culross. Depending on time, the palace may be closed for the day but this is still and interesting place to walk around. It was used for many scenes in Outlander. Later in the afternoon the worst of the visitors should have left! Cross the Forth Road Bridge for Edinburgh.
Stay on the A85 from Lochearnhead through Comrie an attractive small village and a possible coffee stop. Have a look at this short walk or there is this one.
Crieff, larger but equally attractive is a good place for lunch. Drummond Castle Gardens off the A822 are one of the best formal gardens in Scotland. The view from the top of the terrace is jaw dropping. (The castle isn't open and there isn't a cafe). Allow plenty of time to walk and enjoy.
Back on A823 continue through Gleneagles (of golfing fame) to the wonderfully named Yetts o'Muchart to Dunfermline with its Abbey where many Scotiish Kings were buried, including Robert the Bruce. The ruined abbey is in the care of Historic Environment Scotland and entereed separately.
From Dunfermline head to Edinburgh over the Forth Bridge.