That “open first” box should contain basics to help you get by for a day or two. I’d add toilet paper, paper towels, paper plates, and plastic silverware to that box.
Carry a stash of cash for unexpected incidences and expenses. You’ll always get them. Or else have a credit card with extra room on it to charge this sort of expense.
Get refills of any prescriptions you have now. It will be one less thing to have to worry about right away when you get to Wisconsin.
If you’re driving a car there, get the car checked over now, and take care of any problems that you’ve been putting off if possible. Tell the mechanic you’re driving cross country, and get the oil changed, belts and hoses checked, transmission serviced, etc. Breaking down in the middle of nowhere with 3 yowling cats and 2 hungry kids is no fun.
It seems to me that Wisconsin had some sort of weird law about registering a car from out of state. They want you to pay sales tax on the original value when you bought it, rather than current value, or something like that. Contact the Wisconsin DMV to find out. I may be all wrong on this, and maybe some WI dopers can correct me or clarify this. I’d hate to get hit with a big bill like this when you expect to pay only $100 or so to get your Wisconsin tags.
Carry a notebook, and make a list of all the things you have to do - utilities to shut off, banks and insurance agents and doctors to notify, that sort of thing. You always think of them when you’re out doing something - this gives you a way to write it down before you forget it.
If possible, color code the boxes according to what room they go into. You don’t need anything exotic - labeling them with a colored marker when you write the contents will do. It will make things simpler for the movers to unpack at the other end if they know “Blue is the bathroom, red is the kitchen, black goes in the corner at the back of the living room”, etc.
If you have extra fragile stuff to pack, like great-great-great-grandma’s china that came over on the Mayflower, consider letting the movers pack that. They do this for a living, and if it breaks, it’s their responsibility. Or at least get their input on how to pack it.
Talk to your vet about how to transport the animals. Dogs usually love car rides - cats, not so much. A vet can prescribe something to tranquilize a pet and make the ride easier. Three days of driving with a yowling cat would get on anyone’s nerves. But think about trying the tranquilizer on the animal well ahead of time. A friend got some from his vet for his cats, and gave it to them just before a 12 hour drive. Turns out one of the cats was allergic to the stuff. Again, consult with your vet on this!
The suggestion to thin your possessions is a great one. The last time we moved, we moved way too much stuff and junk, and we only went 30 miles. I would hate the idea of trying to move now, and we’ve only been in this place for 14 years. Consider 3 groups when you’re sorting - one for things you use every day, and have to have. Next is things you use reasonably often, or are hard to replace, or great sentimental value. Last is stuff that gets used once a year, or is easy to replace, or is worn or damaged, or is bulky and hard to move. Group 1 goes without question. Group 2 probably goes, and group 3 probably gets thrown away or sold or donated.
Oh, one last thing - apparently, movers expect tips at the end of the job. When we moved, the 2 guys hung around forever when the work was done, and I couldn’t figure out why. I offered them beer, and that was accepted well enough, but they waited even longer. I only found out why later, probably after I was cursed and badmouthed all the way back to their office.