Activity: Buy her one of her favorite sweets (cupcake, candy bar, etc?) and give it to her unexpectedly with a corny note saying something about how sweet she is.
This one worked out good because it was my youngest's 1st birthday. My wife had to work till 7:00 that day. When I got off work I picked up my son from the babysitters then we went and got some of her favorite cupcakes. I took them up to her work and suprised her with cupcakes and her little birthday boy.
She was so suprised and happy that she got to spend a little more time with her baby on his birthday. Usually when she works till 7:00 she isn't home till 7:30 and once she is home we give him a bath and put him to bed around 8:00. So getting to see him a little bit extra really made her day.