I would suggest you take the bus from the airport to Brussels-Luxembourg station - you don't have to transfer in North Station this way and it's only 3 EUR / trip if you buy your ticket at the vending machine (5 EUR with the driver). Train is 5,05 EUR single.
Plan your excursion to Bruges and Ghent on weekend days: this way you can get a weekend ticket to either City. (13,80 EUR r/t to Bruges, 9 EUR r/t to Ghent). You will have to transfer at North, Central or South Station in order to get there. If you don't mind the walk, hop over to central station to catch your train.
For travel within Brussels (not the airport!), you can can buy a day pass for 4,50 EUR.