Exciting, exciting, exciting! I wish I could go with you. I do love Scotland and want to go back there someday.
First tip: Here is my blog from my UK trip:
http://humongousukadventure.blogspot.com/2018/05/ Start looking at the end of May.
Second tip: I assume you are flying into Edinburgh, so head down the main drag (aka Royal Mile) and look for a Carphone Warehouse.
https://www.carphonewarehouse.com/ You can buy a UK SIM card for your cell phone and have cheap cell service, and more importantly, data service so you can use maps and such when you get lost. I bought one good for a month and can't remember how many data Gigs and tons of local UK calls for 15 GBP. Much cheaper than using your US phone. Mine also had unlimited texting anywhere, so I could text home to the US. Just go in and tell them what you need. Having maps and such on your cell phone will save you lots of wandering around. You can call taxis, call hotels, and ask for directions and a ton of things. You want a pay-as-you-go SIM card.
Third Tip: In the UK, us old folks are known as "concessions" so look for concession prices on entrance fees!!
Fourth tip: Use the official tourist site,
https://www.visitscotland.com/. There are lots of inexpensive bed and breakfasts, which are better than a hostel and you will have your own bathroom. Nearly all will have free internet, so you can keep on posting here.
Fifth tip: Pack a washcloth. They don't provide them so I had to buy one at Boots, the UK version of CVS and Walgreens rolled into one. And try Pret a Manger for fantastic organic takeout with very fresh ingredients. For example, a ham sandwich has slices of baked ham, not the lunchmeat ham. Ditto for a chicken sandwich having big chunks of chicken breast. Lovely fresh-squeezed OJ in the morning also.
Are you renting a car? I know you will only be there two weeks, but it is worth it for maybe a week to get out into the countryside. If you can handle an RV with a pile of dogs in the back, you can drive on the wrong side of the road!
And if you really want an adventure and to impress your grandkids, you can take the Hogswort's Express, aka Jacobite Steam Train out of Mallaig. If you arrive to the station early, they will let you sit in the train engine and pretend you are driving it. (Engine is different, but passenger cars are exactly the same as they were in the movies.)