Have a brilliant time in Glasgow! There's plenty to do up there, depending on your interests. A good explore of the city certainly would be good fun. I don't know if the transport museum near Partick is open currently, I've not been yet but I'm assured it's worth doing.
On a trains day, a ride through the low level lines at Queen Street and Central on a 318 or 320, in a motor coach of course, is rather good fun. The SPT Subway is of course highly recommended, especially given there's new trains on the way. ScotRail also have their 153s converted into cycle trains, and while I wouldn't suggest paying the supplement within Glasgow (£10 is a lot of money, especially if you're not taking a cycle with you, on such a short journey!) then if you get the chance to have a look inside one then I'd go for it. If only to see how much tidier and nicer 153s can look!
Of course, if the option is there, a journey on a ScotRail HST would be highly recommended. I have, sadly, not yet done one with ScotRail but it is firmly on the list to do. Even if it's just as far as Stirling.
I do also recommend the 385s, I think they're great trains. Only ever done them in Standard Class, not looked at First Class on them but I would imagine they'd be nice enough. I'd certainly not complain at having one from Glasgow to Edinburgh, indeed I did Haymarket to Glasgow Central via Shotts on an express working in October 2020 on my first ever 385. There's a story to tell there, but I'm keeping hold of that for a wet day that I'm not using to plan a cycling holiday!
Back to Glasgow, it may not be your thing but there's plenty of canal towpath walks available. Kelvingrove Park, and National Cycle Network Route 754 through it is definitely recommended, and there's train/Subway options at both ends Of course, you also have the waterfront to walk along, which on a nice day is a good one to do.
I hope this helps, sorry it's a bit late to be sharing all this but if it helps make the day more interesting, or gives you ideas for next time, then it's all good

Whatever you get up to, have a great time!