I am going to be in Brugges - can anyone suggest a day trip maybe into the Netherlands?

I would rather suggest a trip to the battlefields of the first world war.To start visit the great war museum "In Flanders field" in Ypres.

tovarich has made an excellent suggestion.The museum is terrific and you can pick up a battlefield tour from there. The town itself is pleasant to stroll around

Depends on your interests. You can also rent a bicycle and have a tour in the 'polders' north of Brugge, visiting 13th C Damme or Sluys just over the Dutch border, riding alongside the canals even till the sea. It is not very far. Brugge Damme is some 5 km, Damme-Sluys maybe 12 more. Generally safe roads, some 99% with bicycle paths.

Are you only staying in Brugge? Lots of places you could visit in Belgium; Antwerp for starters. Or Gent, even closer to Brugge. Or Brussel. Suggestions above are good too.
If you're there in summer, visit Knokke. Rent bicycles, cycle to the Zwin nature reserve. Go shopping.

Cycling in and around Bruges is a good idea! I do not doubt that the pictures and the itinerary displayed on http://www.everytrail.com/view_trip.php?trip_id=670865 will convince you ;-)

I highly recommend Quasimodo Tours. Reputable and knowledgeable. Was there in Oct and took triple treat tour. Saw countryside, got history lessons and still remember how good the waffle was. They also offer Ypres battlefield tour. Would have taken it, too, but didn't have time.