First things first....Curacao is not your typical island. It is made up of rock and desert scrub. There are very few sandy beaches, and what few there are, are man-made.
IMO...the Number 1 thing to do is right across the bay from the ship.
You don't need a guide. You walk across the floating bridge (which is very cool, because throughout the day, they open and close it to let boats go through.
The main shopping district is here. It is the group of colorful buildings that you see in every article about Curacao. There are restaurants, bars, store where you find the usual tourist stuff, and also local crafts, Delft, jewelry, and the liqueur, curacao. You can even get free samples sometimes. There are approx 3-4 streets of shops, small museums & galleries. Also right here is Fort Amsterdam, the Government Buildings & the Church museum. Very interesting! On the corner of one of the main streets is a store like a Circle K where you can re-stock whatever you may need halfway through your cruise.
The floating market is right here also. Some amazingly small boats make their way from Venezuela every day with fruits, vegetables and fish. Very busy place early in the a.m. and very interesting.
On the same side of the bay that the ship is on are also several museums and galleries that are interesting to see.
All of this is free, except for small donations at some of the museums.