I have two -
1) Bubbles - simple and hours of play. My son couldn't have a party this year so I purchased bubbles for each child in his class. I had parents thanking me for the gift for weeks.
2)PJ - let me explain because this one can be pricey - my daughter had a sleepover party with 3 friends, instead of buying "Goodie Bags" I bought each girl matching pj's. The pictures I took were cute and each girl has something that will remind them of the night for at least a year...more if they have a younger sibling.