In Groningen train station, there are luggage lockers where you can store your luggage safely. Groningen is the main railway station in Groningen in the Netherlands. It is located on the Harlingen–Nieuweschans railway between Zuidhorn and Groningen Europapark, on the Meppel–Groningen railway as terminus after Groningen Europapark, and on the Groningen–Delfzijl railway as terminus before Groningen Noord. The first station building was completed in 1865 and demolished in 1894. The second and current station building was designed by Izaak Gosschalk, completed in 1896, and most recently restored in 2000. Train services started in 1866 and are currently provided by Nederlandse Spoorwegen and Arriva. There are 41 bus services at the station provided by Qbuzz. The luggage lockers are secured with an electronic code. Take good care of the locker ticket, because you will need it to open the locker again and to pay when you collect your belongings.
The infomation below is from the NS website.
Small locker: 90cm deep, 45cm tall and 35cm wide
Large locker: 90cm deep, 60cm tall and 35cm wide
0 - 24 hours : € 3.85
25 - 72 hours: € 5.55 per 24 hours
0 - 24 hours : € 5.70
25 - 72 hours: € 8.45 per 24 hours
Longer than 72 hours: € 11.25 per 24 hours
Monday 6 AM to 2:00 AM
Tuesday 6 AM to 2:00 AM
Wednesday 6 AM to 2:00 AM
Thursday 6 AM to 2:00 AM
Friday 6 AM to 2:00 AM
Saturday 6 AM to 2:00 AM
Sunday 6 AM to 2:00 AM