We spent a month in Scotland from mid-July to mid-August...
This year's Tattoo event -- my first -- was phenomenal. We sprung for the Jacobite package, which included dinner in the castle and then seats in the "U" part of the horseshoe with a direct view of the castle walls on which they project amazing lighting. Worth every penny -- but not cheap. We did the Fringe, too, the shows for which ARE relatively inexpensive so we saw six to eight performers. I wouldn't go to Edinburgh JUST for the Fringe, but since we were there, it was a break from history. Entertainers worked hard. And the city was CROWDED. Your lodging's fee will be adjusted upward accordingly. And if you want more than pub grub for dinner, plan ahead with reservations...
We visited Culloden when we stayed in Inverness. I agree with a previous poster that the drive is too much of an investment for your "return." Your best bet for a battlefield visit is Bannockburn. But I enjoyed both and found the guides and presentations at Culloden to be fantastic. Speaking of guides, we tagged along with the free guides provided at Stirling Castle and they were also fantastic. We enjoyed that stop enormously.
I was -- at best -- warm about stopping to see the Kelpies. My mind was CHANGED instantly upon seeing them. Interesting mixture of art and engineering. A friend sent me some amazing video of 96 floating candles in the pond around Kelpies in honor of the Queen's passing. (Google that.)
We did the one-day, three-island tour out of Oban - Mull, Staffa and Iona. You can pre-book online before you go, which we did. Rick suggests the first boat out. It was fully booked, but we didn't feel like we missed out on anything with the second trip out of Oban. I'm not a big fan of riding on a bus with 60 other tourists, but it was fine... In fact it was very comfortable and enjoyable. The driver was entertaining and informative. We were there at the end of July -- and the weather was stunning. The waters barely had a ripple. It was so amazing the drivers joined the boat to Staffa because they'd never seen it that amazing. We enjoyed the PUffins and the caves. And the boat ride out and back we were thoroughly entertained by a pod of Minke whales and many porpoises. Take your own water and sandwiches if you'd like (you can grab them in town before you go to the boat), but you can grab food and drinks on the trip.
A side note... Fewer than 12 hours before we were to arrive our VRBO in Oban canceled because the hosts had Covid. Panicked because it is the high season, we ultimately found a lovely room in Perle Oban... Not cheap, but it was among the most memorable settings on our journey. Great views of the harbor, and easy walking to pubs and eateries. The meals at Perle Oban are also quite good.
A bonus on our return from the three-island tour: the Oban High pipe and drum band plays on Wednesday evenings as a fund-raiser for folks in town and and those traveling through. They were great fun. We found out later, the teacher is one of the top five pipers in the world.
Then we spent time in Skye at a VRBO on the Waternish Peninsula and we had nary a crowd on Skye. It was probably the best mix with the locals for us. Walks along the road gave us a handful of opportunities to speak with people who've lived their for decades or even their entire lives. We stayed at a self-catering place on Geary on Isle of Skye (use google maps and zoom in to see the various self-catering places on Geary Rd (I thinks its a road). From there we took GREAT hikes, including one from Trumpan Church out to the Clan MacLeod Memorial Cairn and Dun Borriafiach - an iron-age defense site. Wear water-proof boots because it gets boggy there. Loch Bay was lovely - and crowd-free, too.
We hoped to take the Cal-Mac ferry from the mainland to Skye, but staffing issues caused the company to cancel an entire day's runs. We wound up using the Skye Bridge. Not horrible, just more driving.