Stirling/Wallace Monument/Falkirk Wheel are quite close to each other, can be done as a day trip from either city, but as a threesome probably slightly more logical from Edinburgh. The Wallace Monument is in Stirling so it's only really two stops.
From Glasgow I would suggest doing one of the Clyde Islands- all very easy by train.
Arran- by train to Ardrossan Harbour- which all connect with the adjacent boats over;
Rothesay (Isle of Bute)- Train to Wemyss Bay (the station is an important piece of transport architecture itself) and the boats leave from the station itself-
Cumbrae- train to Largs- very short walk to the ferry terminal- buses on the island for the short drive to the Island 'Capital'- Millport. Largs itself has several wonderful Italian restaurants and ice cream parlours
Dunoon (the Cowal Peninsula, not an island)- ferries from Gourock Railway Station - a couple of good museums in Dunoon and easy bus ride to the excellent Benmore Gardens, or ride down to Toward lighthouse (use the West Coast Motors App to purchase tickets).
All the above are trains from Glasgow Central station.
Also easy to train to Balloch and take a boat ride on Loch Lomond.
All are frequent trains and super easy.
Also from Edinburgh think about taking a suburban train to North Berwick for the Scottish Seabird Centre and wonderful beaches.
Or a train down into the Scottish Borders- at Tweedbank and Galashiels.