Learned this the hard way dragging luggage around for 6 hours. You can book lockers online through the NS app up to 24 hours ahead. €6-12 depending on size, way cheaper than hotel storage fees.

Alternative is left luggage services at some hostels even if you're not staying there. Hostelle provides free luggage storage before check-in and after check-out until 22:00, which beats wandering around with a backpack in 30-degree heat.