Since Mother's Day falls right in the middle of spring, why not celebrate outdoors? If the weather's nice, take mom to a local park for a relaxed picnic lunch. Bring frisbees, kites, or other fun toys for the kids.
You could also visit a zoo and check out mother animals and their babies, or be active and go on a family bike ride and see who can spot the most wildflowers (be sure to leave them to keep growing so other moms can enjoy them, too).
If you're looking for a more indoor idea, you can take mom out to brunch at her favorite restaurant.
If your town has a tea parlor, that might be the perfect, relaxing afternoon treat for mom.
You could also stay at home and cook mom her favorite dish, but make sure she knows she's not allowed to help cook or clean up after!
If you do stay home, you could throw mom a film festival. Make it a pajama party and snuggle up in a pile of blankets with the kids on the floor or couch. Stock up on some of mom's favorite snacks to share during the movie.
As for what to show, you can either pick out some of her favorite films, or try a few of these that mom is sure to love:
Movies to share with the family if the kids are older: Casablanca, The Joy Luck Club, Brief Encounter, Sleepless in Seattle, Calendar Girls, Terms of Endearment, Girl With a Pearl Earring, Steel Magnolias, Chocolat, Amelie, or Gone With The Wind.
Movies to share with the family, including younger kids: The Parent Trap, Freaky Friday, Cheaper by the Dozen, The Incredibles, or The Sound of Music.