Utrecht is close enough that the transfer barely counts as travel, and calm enough that it functions as an actual recovery day rather than a second city to conquer.
The Wharf Cellars and Oudegracht
Utrecht's canals sit a level below street level, with the old wharf cellars now occupied by cafes and bars at the water's edge. Walking the Oudegracht at a slow pace is close to zero-effort sightseeing.
Dom Tower
The tallest church tower in the Netherlands dominates the skyline and is visible from most of the old centre, useful as a fixed point for a wander rather than something requiring a dedicated visit on tired legs.
Where to Stay and Eat
The centre is compact enough that any hotel near the station or Oudegracht puts everything within a short walk. Restaurants along the wharf cellars suit a slow dinner without much walking beforehand.
Getting Back
Trains return to Amsterdam Centraal in around 25 minutes, frequent enough that timing the return around a flight or onward plan is straightforward.