There are not many wet weather options, other than going out in the rain. But it's not meant to be too cold, so that's probably going to be OK. Have you got a nice place to stay with decent amount of indoor space and drying facilities? If so, I'd still go - it's really lovely up there.
If weather is mixed, try going to 'beaches with a walk'. Or walks with 'throwing stones places' on the river or lochside.
There's a small swimming pool at Poolewe. Opening hours are limited and it's got a small capacity - look it up to find out how it's working under covid.
There are some cafes - Badachro, Gairloch, Poolewe (?), Inverewe, Kinlochewe.
There's a pony trekking place (by the harbour).
If you're up for 'a drive with scenery', then Torridon & stop for a walk at Loch Clair.
See if Big Sands has got wet weather ideas on their website?
(This based on several holidays in the area, but not been for a couple of years now).