Well let me firstly advise you the cruise ship doesn't actually come to Glasgow. It comes to Greenock about twenty seven miles (43 KM) downriver. The river Clyde that is.
But not to worry it's just on the edge of a fast motorway (freeway) to the City. The drive along the river gives you a chance to talk about and see Newark Castle, Dumbarton Castle (where Meghan is countess) the towns of Greenock and Port Glasgow.
The cruise lines usually offer tours of the area (by Bus) as to be honest there isn't much in Greenock. A local bus company offers an hour trip round the town it's about ten quid, pretty cheap really. But apart from the view at the top of the hill there's not much to see (great view of the Clyde and Ben Lomond). There's some interesting history to be told but not there any more.
So if youve booked a tour it will be Loch Lomond, Stirling and possibly Glasgow back on the ship for four pm. If you're on a Princess line ship there will be twenty buses and you will be in convoy to Luss (Loch Lomond) then Stirling.
This is a real shame as Loch Lomond is well worth seeing. A little boat trip across the loch with tea or lunch on the other side, a chance to see the Osprey's. Some tales of the history of the area in a small group. Just not possible with a coachload of fifty or so guests.
A small group of three or four could be toured for about £100 each.
Glasgow is another great city and to be honest a six hour trip only touches the city.
But a small group could do a city tour. Visit the University, The Cathedral and pub lunch snack in a typical Glasgow pub. All for £100 per pers.
Book a guide.