Here's my €0.02.
Just say no to fanny packs. Say yes to a small cross-body purse with the strap permanently attached (not with rings and lobster clasps that are easy to open) for things you need handy but can afford to lose.
Wear it like others have said, with your non-dominant hand resting on it. It may not be as pretty, but it's also recommended to wear it with the back out so that the typical zipper pockets are next to your body.
Do not whip it around to the side or to your back to keep it out of the way when you bend over to get a closer look at that perfect tomato in the market. If you feel the need for a small daypack, put even less important things in it. If you use both a purse and a daypack, put the purse on first.
There are many ways to secure things both inside purses and inside daypacks. I use split key rings or little carabiners.
Yes, I also think you need a money belt. I'll also encourage you to carry some kind of emergency information on your body, in a money belt and in your purse and daypack. Identification redundancy is a good friend to have.
I carry my passport in my money belt on my body at all times, not a copy of it. To me, copies are for leaving in the room. The originals are in my money belt in case my purse goes AWOL. Or in case something happens that makes me incapable of communication.
I'm probably hyper about this because I travel solo, am older (although I did it when I wasn't older) and have pre-existing conditions. But being a step-mom to a set of identical twins both of whom had severe strokes in their early 40s (!) also made me realize that anything can happen to any of us at any time.
There are many types of money belts that will work. It depends on what feels the most secure and comfortable for the wearer. I've tried them all and this StashBandz one is the one I like best.
Obviously, you're supposed to wear it under your clothes, not outside them, no matter how fun the color is. For me, the layers go like this: skin, bra, Spanx tank, money belt, panties, pants (I never wear dresses or skirts), top. (Those who don't regularly wear shapewear might be surprised to learn that shapewear camisoles should always be worn under panties, not over them.)
My StashBandz has had a good workout recently because I've had to wear a sensor on a patch that has to communicate with a cell phone monitor. The 2 devices have to be close to each other. When my pockets are too small, I put the phone monitor in my StashBandz.
Good luck with your planning. It's a steep learning curve for the first trip, but just think how much easier it will be for the ones that follow. 😉