I agree that a detour to Maastricht is a bit much, even though the city
is quite special.
The one day on Walcheren (the county of Middelburg) can be very pleasantly spent by exploring the town of Middelburg, the cute fishing town of Veere, a walk on the beach at Domburg, where the breakers can be spectacular. Then, I imagine you take the new tunnel (which I believe is finished now) or ferry from Vlissingen to Breskens. On the way to Brugge you pass the town of Sluis, worth while to visit. There you find one of the best restaurants in Holland (Oude Sluis, 2** Michelin), but I think they might be fully booked on April 1st.
A recommended route to Middelburg is by way of Rotterdam to Goeree, Schouwen and then on to Walcheren. This western route will take you over the Deltaworks, a spectacular waterworks engineering feat after the disastrous freak tide in '53 when most of the province was under water and thousands drowned. There are gigantic movable barriers that will close the estuaries when a storm is under way. I'm afraid the museum will be closed at Easter, but you can find information beforehand at a VVV (Dutch tourist office) to appreciate the Deltaworks. I believe there's a web site. I'll check it out.
Even though museums, etc. will be closed April 1st, restaurants and bars will be doing terrific business. It seems the Dutch nowadays take 6 vacations a year, and you will not be alone!