All the other tips I can provide are quite simple compared to this one.
Think about how you can make your wife feel sexy and feminine. The best solution to this matter is choosing a silk robe, lingerie and expensive perfumes. They will make your wife very happy. Or even better: offer her a weekend away with you, leaving the kids at home, of course. Another option would be hiring a housekeeper. I don't know which of these last two options would be most appreciated by your wife, but you can try both out and see what happens!
If your wife likes going to a symphony concert, play or opera , buy the tickets and absolutely go with her, without any complaint, at least not with her present! You could also pay for some for art lessons, piano lessons, cooking lessons or French lessons, according to her interests. Should she be fond of golf or tennis, give her lessons or a new set of clubs or a racket.
Some people would go wild for a good book, so try this one out too.
Should your wife love flowers, then you must find the most beautiful vase available on the market and fill it with roses. It will make quite an impression! You could also buy a picnic basket and stock it with enough food and wine for the two of you.
The most impressive gift for a wife is by far a new car. That is if you can afford it.....if not, a nice camera will also do the trick!
Never buy gift certificates as a gift for your wife, unless they are for something specific, for example a week at a luxury spa, as they see it as too impersonal a thing to give to a wife. Women are not so comfortable with such a gift as a makeover or exercise equipment, especially coming from their husbands. They usually get the wrong message, so it is best that you go for something else as a present.
To sum it up, when it comes to offering your wife a gift, you can allways rely on art, jewelry, clothes, a purse or chocolates to get you out of such a task.