Anything 'new' would be good - it'll take them awhile to look at it. I would buy a little gift bag for each and put their names on it with glitter glue. Then buy a few things at the dollar store - stickers (unwrap them so they can be occupied during the event without bothering you to do it), a notepad of some sort, a new story - maybe one with the flaps that open, a photo album filled with pictures of people they know, a small doll or other with a bottle so they can 'feed' her, new bracelet, etc. Taking out and putting back in the bag will be half of the fun.
Good luck though - mine are 5 and just turned 4 and I'm not sure how long they'd actually stay in a stroller right now. We haven't used one in awhile and then only to go from point A to point B but standing still.....they wouldn't go for it. I have better luck choosing a place for them to sit over to the side now if it's something I need them to sit still (like at a doctor's appointment of mine or something) with little figurines (pooh, princess, peter pan) and a snack (if you plan this right it takes a lot of time and keeps them very happy- like if they are starting to get hungry right as the event starts). Anyway, I wouldn't plan for the stroller keeping them out of getting up and moving unless they're used to doing that.
