It depends what you are wanting, the routes north of Glasgow and Edinburgh are very scenic with the Glasgow - Fort William - Mallaig and Inverness  - Kyle Of Lochalsh being the most scenic.

1st class is worth it on the main IC routes particularly those to/from London where as well as larger seats and less crowding you can expect some free food service.

Scenic routes sound great! Right now sitting on a train is my favourite thing . So will be doing a lot of that. 

Which one of the endpoints of a scenic route in Scotland is best for spending the evening + night before heading back…? 

Once you have an outline plan I advise you to choose your Eurostar route and reserve seats as they are limited, especially to Amsterdam. I would suggest you consider Brussels as there are far more E* trains to there and there is an hourly Inter City service to Amsterdam. (Avoid the direct Thalys from Brussels to Amsterdam as the reservation fees are very high.

ps. If you want a stop on the way to/from London you should stay in York.

https://www.visityork.org/
 

With a first class pass you can use the first class lounges on the East Coast line from London to Scotland. (Not quite airline lounge but better than trying to find a seat in the station. 

OOI St Pancras (E*) and Kings Cross (LNER terminus) are next door to each other, so you could journey down from York in the morning for an afternoon E* with a 5 min walk between the 2 stations, avoiding an expensive London hotel.

Hallo Yorkie, you are always well informed- we arrive by Eurostar from Rotterdam in St.Pancras at 16.57. We have first class interrail, so we can use the LNER-lounge in King’s Cross? There was some discussion about it. Can we manage to get the LNER 17.30 northbound? Or better the 18.00?

Simple answers for once.

Yes you can use the first class lounge. Plenty of coffee and biscuits but you have to wait for the train for alcohol and food.

I hope you get a better meal on E* than our last journey - dry flavourless couscous but a reasonable wine.

If you get reservations for both trains (free from LNER) you can choose once you arrive. I suspect better you aim for either the 1800 or the following one then you will be unstressed. Do not worry about not turning up - there will always be passengers looking for free seats, even in first class. 

https://www.lner.co.uk/travel-information/make-a-reservation/

It takes between 15 and 30 mins to get from E* to Kings Cross.

The first class lounge has access to a walkway across the middle of the platforms so you do not have to queue with the main bulk of passengers - just go down about 10 mins before departure. The staff in the lounge will show you the shortcut (just along the corridor).

Access to the lounge is via a discreet doorway and a lift up to first floor.

Hello Yorkie, very useful information!! We will travel in about 3,5 week, so we problably will have an other meal on E*! Thank you very much!

I have purchased a pass and booked some trips .Will do UK from Sunday afternoon until Friday, when I’ll be doing Eurostar and a night train to Prague. 
As it’s not going to be that long in Scotland, I thought maybe → Edinburgh on Sunday evening, then → Inverness the next day and a couple of nights there and back to south, spend one night on the way and one in London before heading east. Make sense? Is Inverness a place to spend two nights, or Edinburgh…?  (One alternative is to take then night train to London, although that would mean sleeping in a normal seat, the sleeper fares for the Caledonian are bit steep for me.) 

Edinburgh is certainly worth your 2 night stop. 
On the LNER train make sure to book seats on the right side of the train (ie the coast side) between Durham and Edinburgh (left side on the way back south) as the view of Durham cathedral is beautiful and then the coast up to Edinburgh is stunning. 

Great point! I did already book a seat - I don’t know if GWR gives you the possibility to choose, I just got a random seat (coach: K Seat: 20)

And how can I know if my seat is on the “right” side? 

IIRC you can select your seat when booking on the LNER website. You could check there where your seat is.

I managed to check this out on LNER website, and it’s on the “right” side (right)! Also nice to know there will be complimentary food and alcohol! Easy to make reservation there, too, so next time will be doing it on LNER and not GWR. 

If you return, it might be a nice idea to go back via the West Coast mainline, from Glasgow, with Avanti, the view is also rather pleasant between Lancaster and Carstairs. Especially around the lake district. 

Thanks! What does it mean for long journeys that you get complimentary food and drink? Surely not ”all you can drink”…?

The complimentary first class offer is served airline style by hosts, how much you get varies depending on the crew, some are more generous than others. On a London - Scotland expect at least one food service, 2 cold drinks and tea/coffee. you might get more, especially if there is a change of crew.

It should be enough to keep you going but the portions aren’t huge.

Regarding the sleeper, I wouldn’t personally recommend a night in a seat. The seats themselves are quite hard (they are the same seats you will get on LNER in 1st class) and not comfortable for sleeping in, the lighting is harsh and the ride of those trains is poor.