When I had that much money in my previous save, I'd gift some funds to family that had moved out. When the teenagers move out, give them a hefty "starting fund" to start their new lives.
A vault with a couple of million in it is a great way to eat up funds with bills too.
As mentioned, opening stores or better still vet clinics and restaurants, as they don't have as high a profit margin and also don't require constant tending by the family. (At least I think restaurants can be open without a member of the household being there. I've only played with it once, I think.)
