I don't know any woman who wouldn't jump at the chance to have a kid-free date night or put on some new jewelry or spruce up the house with some fresh flowers. But most of the women I know who are still "down in the trenches" of motherhood--moms with the very immediate demands of caring for young families weighing on them--would rather have a lot of other things. Cook the meals--and they don't have to be anything special, just give her a day where she doesn't have to cook for all the hungry little mouths or feed them take-out or fast food. Don't let her change a diaper all day. Do a week's worth of laundry (and fold it and put it away). Get that household project, that's been on hold due to your schedule, done for her. Take the time this week to pay closer attention to what she does and who she is--all the little things that she does everyday, that you may not notice on a day-to-day basis, but absolutely have to get done to keep you and your children's live running smoothly. Notice the little personality traits that you love but may have forgotten or temporarily lost in the haze of raising a busy family. Pay attention to how she helps you be better--whether that means pushing you where you wouldn't push yourself, or being patient with you where its not easy to be, or simply making you laugh more than you probably otherwise would. Absorb all that little-but-so-important stuff and write it down for her. Show her some affection just because you love her and she likes the attention, and not just because you're hoping to get lucky.
Motherhood is a lot of fun. Seriously, nothing else in my life could possibly make me laugh as much as my kids and the adventures (and misadventures) of raising them. But its also exhausting, patience-testing, 24-hours-on-call, and, occasionally, very tedious work. If you want to do something nice for your wife for Mother's Day, remember that she doesn't clock out at 6pm. She doesn't get a paycheck for all the busy work she does. Most people don't see the results of her work. She rarely gets feedback other than the occasional whining about how I-don't-want-to-wear-this or I-don't-want-to-eat-that. So much of her life is defined by being a mother (and chances are, she wouldn't have it any other way), but she's not just a mom. She's a whole person, a unique individual and chances are that what she really wants is to feel noticed and appreciated for all that she is and does. Bonus points if you know and love her well enough to know in what specific way she would prefer that that recognition and gratitude be expressed. If you can figure that out, you're probably a pretty awesome husband.