Durham- all day or add Newcastle?

To me this is something you can decide on the day. This is a route which has Advance Fares on sale until minutes before departure, even today when there is massive service disruption due to a damaged bridge at Chester le Street.
Many northbound trains starting short at Newcastle.
Advance Fares exist even for short trips like Durham to Newcastle.
The trick is often to change at Newcastle.
So tonight there are advance fares Durham to Edinburgh direct for £34.20.
But Durham to Newcastle is £4.10, Newcastle to Edinburgh as low as £13.10 on Lumo or TPE.
All fares before railcard discount, so for immediate travel with railcard Durham to Edinburgh for under £12 each with a change, or about £22.50 direct.

Thanks Stuart. I saw that massive disruption announcement for today when I was just checking into this. Yikes.
I have tickets for the Cathedral tour and also the Museum. Those will probably take us until after lunch. Then I saw there is a walking tour of Durham- Walkabout Tours- that takes about an hour and starts at 2. Gets good reviews. That might be fun. If it’s not going to be super expensive to buy that day vs. needing to book anything ahead, that would give a lot more flexibility.

I was wondering also, do the return trains back to Edinburgh offer a view of the Northumberland coastline? Would getting seats on the right side of the train be something we should look into? I was able to upgrade our seats to Durham to first class for around £15 more pp. Looking forward to that and having breakfast on the train!

Yeah I’d decide on the day. Durham is small but the tours you have planned will take a bit of time. I do think Newcastle is worth a stop but if you only have a short time then it’s difficult to get much of a feel for a place. The key areas to see are probably the Quayside and Grey Street.

I would really recommend trying to get to Beamish museum if you possibly can. I went recently for the first time in a long time. It’s fantastic.

There's two good points there- the coastal section is North of Berwick. If you've been on the left hand side on the way south, maybe do the left hand side north especially for the different river views from the King Edward Bridge at Newcastle and the Royal Border Bridge at Berwick, then maybe try to switch sides to the right after Berwick.
Also see if there are some good on the day 1st class prices northbound, and get your dinner on the train. Sometimes it's only a £10 to £15 difference.

Easy to spend other people's money.

You might also like to see if you can catch Evensong at Durham or Newcastle Cathedral. Or, in Edinburgh, at St Mary's Episcopal Cathedral at Haymarket.

Personally (while I like Newcastle) I think you could easily spend the whole day in Durham. And if you can manage Beamish, it's definitely worth it and is a half day in itself.

I was at Beamish from open to close and didn’t see everything. There’s so much there.

Beamish looks fabulous, but I just checked their closing hours and with the other things planned to see in Durham and getting there, I am thinking we wouldn’t have but 3-4 hours max. Will definitely keep it in mind for a future trip, though! Has anyone taken the Durham Castle guided tour? I can’t see the dates and times for September yet, but we are hoping to go if we can coordinate it.

I did the Durham Castle tour in April this year. It happened to be spring break for the college, so few students around, but I don't think that made a difference in the tour. It wasn't long - 45 minutes or just over that. It was interesting, but limited. As a working part of the university, there aren't many areas that are shown. You've undoubtedly found the tour description:
https://www.durham.ac.uk/things-to-do/venues/durham-castle/visit/tickets-and-times/whats-on-a-tour/
The areas are all close, so the tour doesn't cover a lot of literal ground. It is worthwhile if there are tour times during your stop.

I took a taxi both to and from Beamish in April. It is a terrific place - I spent all day, literally opening to closing, skipped lunch, and raced between areas to see it all. Definitely save Beamish for a time when you can fully appreciate it.

The Durham Castle tour is worthwhile. My son was a tour guide when he was a student there so we had the pleasure of watching him in action. Because it's a college (ie student accomodation) there's quite a bit you can't see, but you do see the chapel, the Norman gallery, Black Stairs.and the dining hall.

Re Sept, not sure when it starts up again - that would be the first month of the new academic year...