OP, they should do some castle tours. If they are up for it, they can just drive to ones they are interested in (this is what I did and it was a great way to see much of the country), otherwise get them on a coach. A word of caution--some castles are owned and maintained by the state, and others are private. You can get passes that promise entrance to "every" castle, but that's not accurate--you have to look at which ones the passes are actually giving you access to.
The castles are in various states--some are in carefully preserved ruins, some are restored but uninhabitable, and others are fully functioning. I've been to a large number and was impressed by the depth of information provided at each about the history, architecture, etc. A few favorites:
Caerlaverock. It's in ruins, but in its day was a stunning modern triangular fortress near the English border. The history is fascinating--lots of border clashes and raiding.
Stirling. Way up on a hill, also near the border, lots of amazing history, and was the primary home of the Stewarts until the kingdoms were united.
Doune. It's been used in a number of movies/shows (including Monty Python and the Holy Grail) because it has a very classic medieval fortress look.
Blair. Privately owned, fully restored and functional, it looks as much like a modern mansion as a castle. Really opulent.
Dunrobin. Looks like something out of a Disney movie.
And they should spend some time in Edinburgh. Edinburgh Castle is amazing--HUGE and the center of the city. If they can handle a walking tour, you can also do a nighttime tour of old Edinburgh, which takes you into the underground vaults and tunnels.
I haven't seen much of Ireland outside of Dublin (one very fun touristy bar/pub with a great story they should check out is the Hairy Lemon), but I did once take a train from Dublin to Belfast and it was just a gorgeous trip--if they need a bit of a break, that would be good way to build in some rest. And they should spend time in Belfast if they can--I'd recommend using it as a stop between Ireland and Scotland.
I hope they enjoy it. I absolutely love the isles, Scotland especially.