Honestly the only thing I ever did was attend a bible study that had child care once a week. It was my only reprieve. It was a MOPs (Mothers of Pre-Schoolers) I don't know if they have them now, but it was a whole morning. Coffee and visiting first, then crafts and then a quick parenting teaching. I went for the whole five years of her preschool and really cherish it now. I didn't attend that church either, it wasn't a requirement.
My girlfriend has her 2 children go to a Wednesday Awana's class at a church and they tell the parents to go have a date night. So she does that and loves it. She has no one to help her either.
I don't remember how old your child is, but maybe you could find something like that.
But really, don't count on your MIL, like Bill said, it's only going to drive you crazy -and cause marital problems. Who knows what buttons the bil is pushing to get her to watch his.
I really didn't trust anyone with my kid, so I just grinned and bore it.