I'm not sure if there's a place where these things are listed, but it seems that there is always a TV show being filmed in the East Village (I work nearby.) Recently this has included: several Law and Orders, Smash, Damages, CSI-NY, etc. Very often they're on the east-west streets between 1st ave and Broadway (about E. 3rd st to E. 13th st.) The East Village is a fun neighborhood to wander around anyways, and any time you see a large collection of big truck/vans, trailers, and generators, there's usually something being filmed nearby.

East Village is also great for good deals on lunch specials: Thai, Falafel, Sushi, pizza, noodles, and more. (Deal meaning $10 or under including tax and tip.) Also, not the most amazing, but Two Bros pizza has super cheap pizza -- $1 for a plain slice. Not the most amazing pizza by far, but $1! You can get two cheese slices and a soda for $2.75. They're all over the city, and great for a quick, super cheap lunch.

You should be aware the NY is much colder than the UK and while the end of March is technically spring - you can et a temp of 50 or a temp of 30 and snowing (The enormous blizzard of 88 was at the end of March.) So be sure to have them check the weather forecast before they leave - to be sure they have warm enough clothing and waterproof boots or shoes.

Also - the trees won;t be in leaf yet -- or the flowers out - so Central park will be kind of dreary.

One of my favorite things to do in NYC is to take the subway to Brooklyn and walk back across the Brooklyn Bridge. It's a great walk during the day and possibly better at night with the skyline all lit up. I'm going to leave it up to one of the New Yorkers to say which is the correct stop in Brooklyn, because I don't want to send you the wrong way.