suggestions gladly accepted!

What time of year is this?

If you love islands, how about Mull with a day trip to Iona? Alternatively there is Arran, easily reached by ferry from either Ardrossan, or Claonaig. Or smaller and less rugged is Isle of Cumbrae

On the Island theme-

When you went to Islay did you also go to Jura? That can be accessed from Port Askaig (Islay) or direct from the mainland.

It will cost you but you could even do a day trip to Jura from Balloch (Loch Lomond, near Glasgow) by sea plane- https://www.lochlomondseaplanes.com/jura-day-return

That is an experience.

Or maybe the garden island of Gigha. You can make that an adventure by flying Glasgow to Campbeltown then taxi to Tayinloan port one way, then Citylink bus back (the long way round via Loch Lomond or the short cut from Tarbert (Loch Fyne) to Portavadie (Cowal), through Cowal to Dunoon then ferry to Gourock. These may be little visited parts of Scotland by overseas visitors, but will richly repay you.

Or the Isle of Bute (one way from Wemyss Bay, return via the Colintraive ferry on Cowal, up to Dunoon, for Benmore Gardens then ferry to Gourock)

If you want to stay on dry land Wasleys and I are always promoting the Scottish Borders or Dumfries and Galloway (and up into Ayrshire)- there are a number of posts on those areas. By car an hour or thereabouts down the M74 to Lockerbie for D and G then cut through to Dumfries (or off the M74 at Moffat and the A702). By train under the hour to Lockerbie for connecting buses outside the station to Dumfries.

If you choose Arran, while there is a good Island bus service, you can rent a car from the ferry terminal, or arrange escorted tours through Arran Motors.
You can even visit an Isle off an Isle by going to Holy Isle, Arran.

With two nights you could even visit the Outer Hebridean islands of Lewis and Harris by flying from Glasgow or Edinburgh to Stornoway, in just over an hour.

The world (or Scotland) is your Oyster away from the usual destinations.

Another island idea- fly Glasgow to Tiree. Rent a car at the airport. If you want to there is a ferry the other way to Oban, for train back to Glasgow.
The car can be collected or dropped at the Ferry Terminal- https://tireecarhire.com/

Tiree is another undiscovered gem of an island.

Yet another possible island adventure is to fly Glasgow to Benbecula then rent a car from Car Hire Hebrides then drive down through Uist and Eriskay to Barra, for the bucket list flight back off the beach to Glasgow (or the other way round).
There are any number of permutations on that plan (you could even go north to Stornoway on Lewis) as Car Hire Hebrides also do pick up and drop offs at all the Outer Hebrides airports and ferry ports (Stornoway, Tarbert (Isle of Harris), Berneray, Lochmaddy, Lochboisdale, Eriskay and Castlebay (Barra).

stacidar, it would be helpful to know what you like to do when traveling to Scotland. Are you hikers? Or more into looking at scenery and drinking whisky? Also, will you be renting a car or using public transportation?

That said, I would second wasleys' recommendation for Arran or Mull, but especially Arran. The island is very easy to get to--you just fly into Glasgow, then take the train (or drive) to the Ardrossan ferry, and ferry over to Arran. It is a really lovely island with some great hiking, ancient sites, fun shopping, interesting castle and gardens, several whisky distilleries and ruins. It's a small island so you could see a lot in 2 days, and if you don't have a car, Arran has one of the better bus systems of the Scottish isles, so it's easy to get around by public transportation.

I really enjoyed my stay here in May of 2023, and would recommend it highly. https://www.visitarran.com/

@stacidar- what did you eventually decide to do with those spare 2 nights?

We ended up taking the train to Oban and ferry to Mull. Beautiful scenery. Definitely would rent a car next time around to be able to see more of the island and book a day trip to Staffa. Thanks for all the suggestions!

You might consider the coastal town of Stonehaven, with a trip to the nearby Dunottar Castle as a highlight. Absolutely gorgeous, and also fascinating history.