+1 on booking this year.
This list is not extensive or complete, @beamers may know a few more places properly North East too.
On the way up: Loch Ard is a lovely explore, pretty sheltered, paths all round if needed.
Killin, Loch Tay – launch next to hotel (small fee to outdoor shop) and potter down the (flat) river to the islands and sandy beaches. Visit the castle with the hanging mound and tree…
Aviemore with a packable boat – Loch an Eilein should be lovely. I’ve not paddled there, only swum. Loch Morlich would be busier, but is a nice place. No islands though. @scotroutes must have paddled there.
Loch Maree – a bigger undertaking, but stick to the shore along from launching at the hotel and you can ‘hop’ between islands, rather than doing a big crossing. By then the birds should have finished nesting and you could land.
Loch Oich – there is a lovely day trip from Laggan Lock on Ceann Loch, down the canal, down Loch Oich (stick to the shoreline for shelter/short swim if it goes wrong), down the Canal again to Fort Augustus. Cycle back the way you came – we locked our boat up to a fence at the top basin.
For sheltered sea paddle – I was taken by Sheildaig bay last week. Just don’t get to close to the sea eagle nest…
After that there is a multitude of smaller places, a few sea paddling places (need more tide awareness, but Loch Sween in Argyll is lovely) etc.