The Tugboat is both a signature dive and snorkel site. It's 15' deep. The wall to the south is a good dive but can have some current. Under the pier is also good if no ship is tied up/working the area.
Varsenbaai was a pretty good dive. Also a locals beach. Big pontoon boat in 80', we also found seahorses there and turtles along the beach feeding in 6'. A few years ago the operator said they're always there. Check with Trunk Divers on site.
Porto Mari is a nice locals beach. Can be hard to find parking on weekends. And an excellent double reef dive. Should be a decent snorkel to the south. Probably be fairly easy to find a buddy there also - we were asked. Check with Porto Mari Watersports for rentals etc.
They snorkel among the rocks on either side of Playa Lagun. There's turtles out there but probably deeper. Nice beach area also.
5mins. north Playa Jeremi's north side was named one of the top Caribbean snorkels by Frommers a few years ago. No facilities there at all. Both are also good dive sites - get tanks for either at Discover Diving in Lagun. Plan on 30+ mins. drive from Atlantis.
The shopping district downtown is world famous - try to catch that on a day when less/no cruise ships are in. There's a farmers/fish market that sets up right along the waterfront on the Punda (east) side. And the world famous swinging bridge, walk over it to Rif Fort - lots of shops/restaurants etc. there as well. Also the cruise port is just north on that side. Somewhere there's a cruise arrival schedule online for Curacao. Punda was really crowded the day we were there when two ships were in.
Ribs Factory in Salinja had good food. If you're gamblers, there's at least a dozen casinos - most larger hotels have one. The biggest/nicest is probably at the Renaissance - someone told me it's like one you'd find in Reno. Mambo Beach Club for non-kid entertainment. Blues Bar at the Avila Hotel waterfront has live jazz/blues some nights. I think it's near Atlantis.
For kid related things, the Seaquarium is worth a look. And the Ostrich Farm. For big kids with big budgets -
http://www.substation-curacao.com/