There are quite a few things you two can do for each other.
1. Going out, with the diaper hidden or out in the open, going out messy or clean.
2. If you're super inclined get a stroller for you or her. That's embarrassing, and will draw a serious amount of attention.
3. At home, get in the shower(if you use disposables) and fill up the entire diaper. Sit around in it, squish it. She can find some use for it I'm sure.
4. Forced enema's, and pushing the messy load into the diaper.
5. Using the diaper until it literally starts to leak, might recommend plastic pants or some liner for that one.
6. Warm oatmeal down the front or back of the diaper.
7. Cut the top plastic lining inside the diaper that secures the plastic fluff or polymers, and use the diaper like that. It will be messy for sure.
8.Fill the diaper up, with whatever, natural or otherwise, and have her push you down on your butt, on the floor, the tub or in a chair, and the mess will spread out, the sound will be good, and the diaper may break.
9.Kinda gross, but she could wear a diaper, soil it, then force you to wear it. Only if you two are into that.
10.If you two are into face-sitting, do it with diapers.
11.Strap one diaper one, cut slits in it, and then put another diaper on over it to maximize the time and mess spent in it.
12.Laxatives to lose control too. Sneak her, or her to you, castor oil somehow, and it will...clean the system out depending on how much is utilized. Be careful, use small amounts to determine how it personally affects you first before trying larger amounts.
13.Banana's inserted up there to push out into the diaper.
14.Using a few frozen marshmallows with lube, Vaseline or petroleum jelly works well inserted up there will cause incontinence until they come out. Diaper up for that. (There are videos online for banana enema's and the marshmallow trick, look em up if you are curious for more info)
15. Spanking in a messy diaper.
16. Blow Job in diaper (Not sure if that's really a punishment unless it's forced, and that may be frowned upon, only a suggestion)