Oops! Tickets to something coming up in your area? Nice flowers or those edible bouquets delivered? Is she local? If so, y'all get dressed up and come over to sing Happy Birthday and deliver her favorite cake, ask her to dinner.
I don't know how long it takes to have a puzzle made from the Walmart photo lab, but we did that one time: made a poster that said "Grandma, we love you to PIECES!" and had a neighbor take a picture of us all with the poster. I ran it down and got it made into a puzzle. If it can't be ready today, you can at least print off a 5x7 and use a glue stick with some cardstock and attach the photo to that, then use a pen knife to cut pieces and make your own puzzle? (Get a cute little tin or interesting box like you'd find at World Market to hold the pieces).
If your kids are old enough to help, you can all pitch in and cut out a bunch of strips of paper (the more the better; I used construction paper for different happy colors) and write out all the reasons you can think of on why you guys love her, what she means to y'all, something cute or special about her. Stuff them in balloons and blow them up and fill her car, give her a card with a large knitting needle that says "use this to see how special you are to us" (and have a nice little box for her to put the reasons in afterwards).
Can't think of any actual gifts that she'd like without knowing something about her.......but you can bite the bullet and buy something she'd like (especially since you forgot until now...which I've done a few times, even with my OWN mom!) :)