I would bring some of the things that have been mentioned already: balls, playing cards, bubbles, coloring books, and maybe a couple of board games.
Some moms have suggested chalk, sprinkler, and slip and slide. I personally don't agree UNLESS you ask your SIL ahead of time. The chalk can get messy and maybe she doesn't want chalk all over everything including clothes. The water activities can create a messy, wet yard. Maybe she doesn't want that during her party. Also... for her young kids... (2 yrs. and 3 yrs. old) a slip and slide can cause accidents. I tried it when my twins were about that age and I put it away until they were 5 because it just didn't seem safe for any younger. (Just my 2 cents!)
I would also not let my kids play handheld games at a party. (Well if it was going to be for the entire day, but not if it's just a 3 hr. party.) In my OPINION... a party is for socializing and too often enough I see kids totally absorbed in these handheld games such as Game Boy and Nintendo DS at times when they could be communicating with others. I understand the idea of letting them use these for awhile at home for alone time, at the doctors while waiting in the waiting room or driving long distances... but at a party I think you could bring some things that they could do with others. Or even if they are coloring... others could join in and adults could start a conversation based on what they are coloring. Teach them some games to play that maybe the young ones could join in too. Bean bag games and putt putt golf plastic clubs would work. Go to Dollar Tree and pick up a few items to bring over. Heck.... 5 items for 5 dollars and it will keep them busy.... I say yes to that! Familyfun.com is a great website for ideas too.
I love the quote that one mom mentioned.... But I have to say that whenever my kids tell me they are bored, I say, "Only boring people are bored." In other words, sometimes, you have to make your own fun.*** I am going to print that out and put it on a wall or refrigerator. But really my kids seldom say that because I try to balance doing thigs with them AND encouraging them to think of things to do on their own (they are 8 now.) When I see them designing, building, reading, drawing, creating etc... I make a postive remark about it to encourage more of it.
Have fun. I also love the comment someone made about bringing certain foods if your child had an allergy. You wouldn't hesitate to bring different food. (Great analogy she made here.)
So don't hesitate to bring some toys to the party but call if you think certain items could be a problem like the ones I mentioned above.
*J.