My recommendation is always to travel light, but if you find your self laden down with luggage then there are a few ways to store your gear while you get out to explore.  The first and most convenient are the luggage lockers (bagagekluizen in Dutch) at the train station. These are at all the large train stations (Utrecht, Amsterdam, Rotterdam, The Hague, etc.). Signage tends to be in English at the bigger stations or you can follow the pictogram. To check in advance if a station has a locker check the NS website here.  Luggage lockers are easy to use and are apparently secure. I say apparently because if anyone tries to break into them they do emit a very loud siren. How do I know this - well one day while trying to force my stuck locker open I managed to set it off - although many onlookers gave me the evil eye and it was quiet embarrassing- no security came and I had to jimmy the door open to retrieve my bag. So ‘secure’ but I guess nothing is 100% secure. ...