You're in for a real treat as Curacao is one of my favorite ports.
First, there are the beautiful pastel colored buildings taht line the canal that goes through Willemstad. This canal is crossed via a pedestrian pontoon bridge that opens using a man in a sort of boat attached to the bridge. This has been used for years and years there. When the bridge is open, everyone uses the free ferry to cross. There are nice outdoor cafes alongside the canal to sit and watch the peoplel and the ships passing through along with the opening and closing of the bridge.
The two best beaches to visit are the Seaquarium Beach that you will need to purchase admission to the aquarium to visit. Others on my ship said it was great. We went to the beach right before Seaquarium (I think it was called Kon Tiki Beach) that had lounge chairs for rent and a nice restaurant with tables under palm thatched areas. This required a minimum admission fee (I think $3)
The shopping is very European and of all the Caribbean Islands, I believe this island gives you the best taste of being in a foreign land. I purchased some great Ecco shoes and sandals that are the most comfortable I have ever had on my feet in the Ecco store. Depending on where your ship docks, the mega liners dock away from the canal, the walk to town is intercepted by a busy native flea market with all kind of goodies.
As far as the night life, 20 years ago, my ship was in port at night and we just stayed on the ship, but there are casinos within walking distance from the ship that you can visit. This last time, our ship left at 6:00 PM.
Have a great cruise!