My husband and I are looking to make our second trip to Scotland towards the end of 2022 (the end of September, beginning of October - we went this time of year last time). We plan to be there for 14 days (2 of course will be taken by traveling to and from Scotland). On our first trip we moved around a lot from city to city and did not get to see as much as we would have liked as we spent a lot of time on trains moving from city to city. That trip was 10 days, so we have increased the length for our 2nd trip. That being said, we would like to concentrate on the highlands area. We took a full day toward of the Isle of Skye on our first trip there, so we will not being visiting Skye on this trip. We would like to see castles, castle ruins, of course some tourist type things, explore a bit of the landscape as well. We would like to have maybe 2-3 "central" locations where we can base ourselves. As I said we do not want to be moving from hotel to hotel constantly as we missed out on seeing a lot in doing that on our first trip. We will be looking into either hiring a car or possibly renting a car. Additionally, we will be using my husband's hotel points for our stay, so that may also somewhat limit us, as his hotel points are for Holiday Inn chain of hotels. We visited Edinburgh, Glasgow, Inverness, Sterling and traveled to the Isle of Bute on our first trip and visited a number of tourist sites in these cities in addition to our all day tour around Skye.
If you’re committed to Holiday Inn/Holiday Inn Express lodging, it appears thee are 19 properties in all of Scotland, many in places you visited on your first trip. The northern tip of a triangle that would include Sterling on the western tip and Edinburgh on the eastern tip, perhaps Perth would make for a central base for exploring farther north and east?
Or if you didn’t get enough on your first trip, are there any places that would warrant a longer, second look this trip, while still staying at a Holiday Inn? Unfortunately, there don’t appear to be Holiday Inns in Glencoe or Fort William, which would otherwise be fantastic cities for stays.
There’s another island group, not Skye, and not mainland highlands, but keep the Orkneys in mind for a Scotland trip. Reachable by ferry or plane, with amazing Neolithic sights, World War II history, and a lot more to experience, plus castles on some norther isles.
Being tied to Holiday Inns is going to severely limit where you can stay. However all is not lost as there are several in and around Aberdeen and Aberdeenshire is prime castle country. It even has its own castle trail, all of which can be visited as day trips from Aberdeen.
Dundee or Glenrothes is another excellent choice as it gives you access to the delightful small town of St Andrews with its ruined castle and cathedral as well as the fishing villages of the Fife Coast. Falkland Palace is well worth a visit as is the ruined Loch Leven Castle in the middle of a loch and reached by boat. If you’ve not been to Castle Campbell, that is also well worth visiting. For scenery there are the Angus Glens.
The final possible is Dumfries which could be used as a base to get into Dumfries and Galloway. This is very much the forgotten part of Scotland as far as tourists are concerned but the scenery in Galloway Forest Park is excellent and Glen Trool is worthy of any of the Highland Lochs. There are also ruined castles (Caerlaverock, Kircudbright, Cardonness) as well as stately homes like Culzean Castle, Drumlarig Castle and and Dumfries House .
The Undiscovered Scotland website will give you lots more ideas of things to do and see in each of these three areas. Start with the relevant map page and follow the links to the text pages with lots of information and pictures. It covers many hidden gems not found in the usual guide books.
Dundee has several worthwhile museums, including the new Victoria and Albert Design Museum. It's easy to day-trip from there to St. Andrews (by bus), and one could probably also pull off a day-trip through the East Neuk villages. That would also be by bus; you could return to Dundee by rail from a convenient point, I'm sure. I believe there's an all-you-can-ride one-day bus ticket that could be useful in that immediate area.
Since nobody else has commented on your phrase "either hiring a car or possibly renting a car," I will. You need to know that in British English, to hire a car is the same thing as what we call renting a car in American English.
If you mean hiring a private driver to take you places in a car, look under "chauffeur" or "private driver" to get an idea of availability and prices (expensive). In some of the more remote areas of Scotland, a taxi driver may basically fulfill this function -- for example, a taxi might drive you from the bus stop to a certain castle you want to visit, and agree to pick you up 2 hours later and return you to the bus stop.
I have been researching Scotland and another options is Rabbies tours. They leave out of Edinburgh, Glasgow, and Inverness. They have both single day and multi-day tours. If you have a car I think Pitlochry and Kenmore look like good stops from which to venture out.