On the Saturday of Easter weekend, I would just take a cab or better yet, a town car service for a few more bucks. The traffic will be light and you can get off your plane, head to the cab line and be at Grand Central in 30 minutes, with any luck. There are train options that will be cheaper, but you will have to wait for trains, change trains, etc. Cab will be quick and low stress. I’m also assuming here that the weather won’t be a factor. If it is snowing or some other severe weather, take the trains.
I frequently use the Olympia Trails bus from EWR to Grand Central. They leave Newark every 15 minutes and drop you a block from Grand Central. I think they’re just about as fast as a cab, although not as exciting. One way is $15, and, if you’re flying Continental, there’s a discount coupon in the in-flight magazine.
The best, cheapest and relatively fastest way to Grand Central (42nd and Lexington) is to take the shuttle bus (I think it’s Olympia but I could be wrong, follow the signs in EWR’s terminal). It’s about $15 one way or $25 round-trip and ends at 41st and Lexington.
Cabs can be gruesomely expensive ($65+). There’s also the train to Penn Station, but then you have to cab or walk it to Grand Central.