3rd graders would love to decorate cookies! I did this for my son's 3rd grade class at Christmas time....we decorated gingerbread boys and girls. I just premade the cookies (although you could buy plain sugar cookies from the store, or make heart shaped ones), bought these foam food trays at
Walmart (like a cafeteria tray....I think a pkg of 40 was like $4) brought in vanilla icing (you could tint yours red or pink) and some candy.
Each child got a foam tray with two cookies and in the compartments I put a small handul of candy each, along with some icing and a plastic knife. I did holidays colored m&m's, sprinkles, gumdrops, etc. But you could do m&m's, sweetheart candy, sprinkles, etc. Really cute, and they ate their craft so it was also a cute snack/party activity.
Some of the girls really got into decorating their cookies. And the ones who didn't want to create masterpieces just had fun building their cookies and eating.
I am going into my daughter's 1st grade class on Thur. I picked up "color your own valentines mug" kits for her class. It comes with four inserts. I am going to have them color one to keep and color one to give to the person sitting next to them. THis way they will have one they created and an insert from a classmate to swap out and remind them of their friendship.
You could also do Valentine's Day bingo and use sweetheart candy as markers. Or another mom in my dd's class made bookmarks with the kids the other day. Michaels has cute wooden picture frames for $1....if you got one each and a big bucket of foam stickes for Valentine's day the kids could decorate a frame. You could take a group pic and then make enough copies for each child and send it back into school the next day.
HTH!