As suggested, drop bag at storage. Well sign-posted and there are also manned info stations with helpful English speaking staff. If the weather is clear, go into town on the Airport Express and take the tram up to Victoria Peak, after visiting the peak take the Star Ferry across the harbor and walk around Kowloon - or - take a blue taxi over to Tung Chung and take the cable car up to Big Buddha. From there you can catch a taxi or bus to Tai O. You want clear weather for either the Peak or the Big Buddha. In fog, you won't see a thing. If a foggy day, take the Airport Express into town, Star Ferry, Ding Ding Tram, wander the neighborhoods. I see you "aren't fussed" about sightseeing, but why not have a hike, good food, what ever else you enjoy.
I wouldn't stay airside. There is shopping, but that will get old fast. Lounges are lounges. You have a very long layover.......... You can always leave the airport and cut short any sightseeing plans if tiredness kicks in.
If you are concerned about protest activity, info desks have been on top of conditions. There are numerous apps that you can use to track anything also.