Re the weather - it's not the Arctic! Seriously I am a born and bred Londoner. The temperature may be two or three degrees lower than London in the summer. But, if you're in the Cairngorms (there is a mountain train now for those who can't walk it), you'll need the suncream.
You don't have to be an outdoorsy type (camping and rambling) to enjoy the countryside. Loch Ness has boat trips onto the loch and the Loch Ness Centre where you can find out about Nessie.
Aviemore has the reindeer centre and a walk up the lower mountain slopes (we had a lady on crutches do it when we were last there so not arduous) - when you get to the enclosure you are surrounded by friendly reindeer that everyone (even toddlers) can feed by hand. It also has windsurfing on Loch Morlich, alongside paddleboarding and kayaking. They have BBQs on the beach at lunch time in the summer. Boat of Garten, close by, is called the Osprey Village because it is home to nesting Osprey which you can see via webcam or telescopes from a hide (we saw beautiful red squirrels when we were there last time) - easy walk even with a pushchair. There's country life museums nearby with people in authentic dress of the period teaching weaving, spinning etc. The Highland Wildlife Park at Aviemore allows you to drive through in your own car with a wolf pack, tigers etc to learn about. Loads of farms do sheep herding shows through the summer and, if you're lucky, some will let you hand feed lambs during the season. There's also a steam train and umpteen distilleries which, even if you're not a whisky lover, is interesting.
At Carrbridge you'll find Landmark Forest Adventure where families can do wall climbs, go on a mini rollercoaster, tunnel slides, rope walks or just have a walk around the forest. They have an indoor 'puzzleworld' type place too if it's wet.
Nairn has Cawdor Castle (home of the Thanes of Cawdor - think MacBeth), with a lovely castle and beautiful grounds.
Edinburgh has Holyrood and Edinburgh Castle, Dynamic Earth (a huge centre showing the development of the earth - very popular with kids as they sometimes have fossilised dino poo so what's not to like ha ha!), Mary Kelly Close (an underground 'city' in Edinburgh) and Arthur's Seat (the extinct volcano) which is an easy walk through a country park type environment.
If you go north to the Moray Firth you'll find the Scottish Dolphin Centre at Spey Bay where you may spot a dolphin or two if you're lucky.
And not a mountain climb or ramble in sight!
We used to fly to Edinburgh then hire a car or stay overnight if driving to break the journey - we're in South East England.
By the way I get bitten every year by gnats in our garden - never had a midge bite.