Sauna is booked on a per usage base, there is no need to book upfront.
Whirlpools are standard in our bathrooms. You can rent the bathrooms for the time of your stay but you can also rent it on a per usage base.
Most of the swimming pools work already with whirlpools, so this might not be a novelty for you.
It is anyway worth while enjoying the relaxation that this can offer.