Here are 3 things that are all relatively close together that you can do in one day.
Take the
Manhattan Architecture Yacht Cruise. It's aboard a vintage-style motor yacht and limited to only about 80 people (unlike the Circle Line which can take 100s). A docent will speak about the architectural history of NYC and point out significant sights. Book the 9:45 sailing so that you'll still have time for the other activities.
https://www.viator.com/tours/New-York-City/Manhattan-Architecture-AIANY-Yacht-Tour/d687-6288P7?mcid=42383&pid=P00082420&medium=api&api_version=2.0&target_lander=NONE¤cy=USD&campaign=%3A03563532-d186-484e-9925-6605a1eb9b79
From there, grab a taxi or Uber over to the
Intrepid. Spend a few hours there and grab some lunch onboard at The Aviator Grill.
https://intrepidmuseum.org/
After that, head over to
The Edge. It's the highest sky deck in the Western hemisphere and the 360° views of NYC and the surrounding area from over 1,000' in the air are incredible. It's a timed entrance, so get tickets for the early evening so you'll be able to see the lights of the city come up at dusk.
https://www.edgenyc.com/en
Most of all, have a great time!