The model train exhibit is put on by the Atlantic Coast "S" Gaugers and is free and open to the public from 11-5. They are a not for profit club that has divisions all over the state. They encourage kids to press all the buttons, and there are a LOT! There is a donation cup if you are willing to help.
Heading back to MacArthur, if you can convince your kid (I couldn't) there is the Impulse exhibit which has sets of large light-up and music-playing seesaws. What was a hit for us though was Ilot De Chaleur, which lights up and will eventually play music the more you run around it. This was really fun.
If you've got time, be sure to check out Dickens' ChristmasTowne also. Doing fun things with your kids doesn't have to cost a lot of money; sometimes it's just finding the things to do! Don't forget- we have a daily calendar of events on our page!