Do you like art? Hit the Met or MoMA. Or a smaller but still world class gallery like the Guggenheim, the Whitney, the Neue, or the Morgan Library. Check their websites to see if any have temporary exhibits right now that interest you

Do you like to be active when you travel?
Get up early to beat the heat and do a jog around Central Park ~6 miles. Or a little lower impact but still fun and memorable: rent a bike (there are several options) and ride around the park a few times

Want to see a show? You can usually score decent tickets the day of the show (I usually pay less and get better seats than you can score at the Eccles in SLC) using various apps or the TKTS booth in Times Square. Also consider: a jazz show at Dizzy's Club, Birdland, Blue Note, etc.

Want to see the city from the water? Consider a circle line cruise around manhattan. Or take the Staten Island Ferry (free). Both give you a nice view of lower manhattan and the Statue of Liberty