you can get $1 packs of glow sticks (braclets) at Michael's, one or two in the baskets, save the rest to hand out in the evening to distract from the vendors peddling expensive glow toys!
Snacks are always good, however, you didn't mention if you have experience with/or kids of your own (I'm leaning towards no, since you are asking for help!

) so, try to stay away from snacks that will be sticky or will melt (like to much chocolate) - of course that depends on how much you like the parents of the kids, I would sometimes intentionally give messy snacks to my nieces just to drive my sister crazy (I would also buy toys that made noise

)
If you like your sibling, stay away from toys with lots of pieces, hotwheels are always good (better yet, get the "Cars" models)
Also, not for in car, but I always pack small bottles of bubbles for when the wait in lines gets too long for little people... I take them even when my own kids aren't with me.. bubbles make everyone happy.. even other people's whining kids