As far as I know cruise ships docks just for one day/limited time in Zeebrugge, so as Kathy already says there is actually no time to visit both Bruges and Brussels. Some cruise companies provide shuttle services, have no idea with yours this time, if not you have to rely on taxies or public transport.
Easy to do and also fun for the kids too is taking the coastal tram to the railway station in nearby Blankenberge. From there the train to Bruges, the historic centre is at walking distance. The tram stop opposite the church in Zeebrugge is some 10min. walking from the cruise terminal and during summer season there is every 10 minutes a tram to Blankenberge (direction De Panne, clearly visible at the frontside). The driver sells the tickets but there is a ticket booth too, but not always open. Good value for 6 persons is buying a “Groepsbiljet”, a ride will cost you €1.25/person instead of the €3 for the usual single ticket. The driver validates the tickets, if not use the yellow boxes at each entrance.
As the trains are operated by another company you need to buy seperate tickets. Return tickets for adults costs €6.20 and children €4.40. Within 15min. you will arrive at the station in “Brugge”. Think about 1h total travelling time from the ship to the centre of Bruges. If nevertheless having the idea of visiting Brussels a train ride from Bruges takes 1 hour, but keep in mind that you will travel on a Sunday during high season so a whole bunch of Belgians will head to the coast and back, so there is always a risk of delays. Take the tram with direction Knokke on the way back to the cruise terminal.
Seen the costs it’s worth considering a minibus (have to say no idea how much they cost), usually they are are waiting just outside the docks along the main road you have otherwise to walk to the tramstop in Zeebrugge.