Pleasant words are a honeycomb, sweet to the soul and healing to the bones. Proverbs 16:24
When you want to learn how to be extra sweet to your wife, just listen to and follow your heart. Bring her flowers, or candy, or buy a card, or write a love note. Keep in mind though, if she thinks she's fat or is trying to lose weight, rethink the candy. She will think you are insensitive and it will only get you into trouble.
If you try the other three up there listed in the first paragraph, or come up with something else on your own, she will think that you are all the sweet she will ever need.
Try some things like cooking her dinner or suggesting you two go for a walk together. If you hold hands when you go someplace she will think that is sweet, too. Talk to her about things she is interested in or ask her about things she would like to try and hasn't yet, then do your best to make them happen for her. Spoil her a little bit every now and then.
Plan a romantic getaway for just the two of you. Make all the arrangements and surprise her with it. A nice warm beach for an extended weekend would probably do the trick. Or maybe a ski weekend where you could spend your days on the slopes and your nights in front of a roaring fire with a couple of nice glasses of wine.
Keep in mind that if you want to know how to be sweet to your wife, something as simple as taking the time to lower the toilet seat every time you leave the bathroom. It may not seem like a big deal to you but she will think it is a sweet thing to do.
At this point, you may be thinking, what is in all this for me. Well, if you do any of these things for her she will recognize them for what they are and respond in kind and do sweet things for you, too. Think of it as a 'win-win' situation.
Do not, however, do sweet things just to try to get a reward and be insensitive the rest of the time. She is not stupid.
If your relationship with your wife stands on solid ground being sweet to her will only strengthen it but if your relationship is a little rocky then being sweet can and will save it from being destroyed.
There is no limit to the sweet things that you can do to keep your wife happy. The main things to remember are, treat her with dignity and respect, never ridicule or belittle her. She is just as important to the relationship as you are. Learn how to be sweet to your wife and she will be sweet to you.