Day Trip from Edinburgh or Glasgow

Or you could leave the driving to someone else:

Rabbies

I personally recommend them and if you do a search, so do many others.

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.

I just returned from 7 nights in Edinburgh and did multiple day trips. They are mentioned in my trip report:

https://community.ricksteves.com/travel-forum/trip-reports/solo-in-the-uk-my-york-and-edinburgh-trip-report

I’m happy to answer any questions you might have about any of them.

By the way I forgot to say earlier that all these are walk up trains. No advance booking needed or even possible.
I know you have been in Wales and not had the best train experience. Generally in Scotland they are modern trains, and of the right length for the demand (or much better than Wales anyway), also they are not changing timetable every time you sneeze as happened in Wales.
A slightly unusual but worthwhile Rabbies tour worth doing from Glasgow is the Culzean Castle and Burns Country tour- it would be a bit hard to put that together yourself.
I can't tell from the Itinerary if they also cover the Electric Brae (a gravity hill which is an optical illusion where you appear to freewheel uphill),
I was actually looking to see if Rabbies did a Trossachs day tour- but I can't see it. That goes out to Callander then onto Loch Katrine (now a reservoir for Glasgow) where you can take a trip on the loch on an old steam ship. Then back via Aberfoyle and maybe Loch Lomond. Without a car you can't sensibly do that. It is a very scenic day trip.
Their Stirling Castle and Loch Lomond tour seems to be the closest one to doing the Trossachs.
If Rabbies don't do it maybe some other international company do that- I know some of the Scottish local companies do.

I loved Stirling and went there as a day trip from Glasgow. I would have liked to have seen the Falkirk Wheel but just ran out of time. I have details of how I got to Stirling from Glasgow in my trip report - https://community.ricksteves.com/travel-forum/scotland/solo-in-scotland-trains-cars-and-ferries-and-a-wee-dram-or-two. I used public transportation (i.e., the train) to get there and then just walked everyplace.

As isn31c said, the Wallace Monument is right in Stirling so it's easy to get to while you're there (along with many other interesting places) but you will have to get to Falkirk. Here's a segment from a post that isn31c answered for me a couple of months ago about getting to Falkirk:

The Falkirk Wheel is easy, in my opinion, from Stirling. Train to
Falkirk Grahamston, then McGills East Scotland bus 6 from outside to
the wheel. The only thing is that the bus doesn't run on a Sunday. On
the way back Bus 6 drops you very close indeed to Falkirk High Station
(see the route map), or you can even walk there along the canal from
the wheel which literally brings you to the end of the station
platform, for the train back to Glasgow.

The cheapest option for the buses is to get a Falkirk plus bus ticket
with your train ticket- https://www.plusbus.info/falkirk (Ignore the
fact it says valid on First Bus services, they were the previous
operator).

I would do these from Edinburgh - but then would cut the days in Glasgow too. The ones you have listed are all fairly easy to do yourself.

In addition, consider a bus down to Roslin for the chapel

We're doing the Rabbies one-day Outlander tour from Edinburgh because my wife loves that show, and its frankly the only way I convinced her to go to Scotland.

fwiw we're taking the train from EDI to GLA with a walking tour and then we'll see what we can on our own, returning the same day.