In-room means a session inside the room or suite where the guest is staying; here it describes every reservation, since there is nowhere else to go.
On a hotel's service card the phrase sits beside dining and the minibar, and it means that something comes up rather than the guest going down. The house uses it in exactly that sense. It keeps no premises, so every reservation it has taken has been in-room: a deluxe double, a club-floor room, a two-bedroom suite or the living room of a serviced apartment.
The spa in a large Amsterdam hotel is a pleasant place within its hours. It has a reception, a treatment menu, fixed appointment lengths and a last booking that often falls in the early evening. An in-room reservation is bound by none of those. It starts whenever the guest asks, midnight included, runs to the length chosen on the ladder and ends in a room the guest need not leave.
Almost any room serves. What varies between categories is space and control rather than suitability. A suite places a sitting room between the door and the bed, which many guests appreciate. A standard room in a converted canal house may be narrow, and moving one chair clears enough floor. An executive-floor room usually brings quieter corridors and a lift that answers only to a card. The guide to hotel room types matches each form in detail.
Very little. Warmth is the one preparation that matters, since hotel climate systems are set for someone dressed and moving about, so the thermostat is worth raising in advance. Blackout curtains drawn and a bedside lamp lit complete it. The therapist brings linens and towels and takes them away again, so the hotel's own stay untouched and nothing need be requested from housekeeping.
No room category changes the price; the same ladder applies to the smallest double and the largest suite. The discretion entry describes how a visit leaves the room as it was found, and five-star hotel considers the setting many guests write from.