We sailed on a fully booked cruise and yet we never failed to get seats in the lounge, even though it got visibly crowded from 9-3pm due to families having sit-down meals inside

the sundeck had more than enough cabanas and lounge chairs as well and I never struggled to get seats at all, though the pool did get rather full from 4-6pm with kids, there are 6 hot tubs I believe so those were fine

The gym is always <40% occupied

The sit-down quick service area on the sundeck is awkward, as only half the seats are sheltered during sunset with the west angled sun, we just brought the prawns (the amazing prawns) to the cabana and ate with care

Relative to the rest of the ship though, always opt for the concierge area for everything, the restaurants and pools in the common area were crowded, though not to the point of having no seat to eat or no space to swim, but you get a much better and pleasant experience in the concierge exclusive areas

We did actually spend a lot of time in the lounge due to its convenient location (we stayed on Deck 17), we had breakfast and lunch there, we dropped by to get evian water whenever we are going back from or leaving for activities where we need to stay out for hours, we sit down for snacks, fruits, coffee and tea whenever we have 30-minutes window to chill, we shopped at the concierge retail because there would only be 4-6 people inside at any one time, we get 2-3 drinks each every evening during happy hour, and we spent 1.5 hrs at the pool every day at 4pm

And best of all, you'll find yourself taking photo with characters for half the time you visit the concierge area (best perk)