I also like the idea of paying her back--though some parents (like mine) would say, "Oh, no, I wanted to pay for your wedding!" and might not take that as a grand gesture but more like a slap in the face. You know your mom best. Maybe take her on her dream trip, whatever that may be (for my mom, it was Europe).
If I were in a financial situation like yours (nearing $1M), I'd offer to pay for the whole trip, and go with her. She may want to spend some of her own funds for souvenirs or whatever, but I'd handle the booking of the airfare, lodging, etc. so she doesn't even know how much that costs.