A Baby Carrier
I used this baby carrier when my babies were newborn to about 3 or 4 months old. It's lightweight, very easy to put on, and can easily be stored in a diaper bag.
A Small Stroller
This was perfect after my daughter was old enough to sit up on her own in the stroller, and my son could scoot along beside me on this. I would usually take the yoyo scooter board attachment as well, but this was nice to give me a break from pushing two kids around the city. If my son decided he was done with the scooter, it can actually be balanced on the yoyo by the handle bars.
I posted this picture a couple months after moving to Paris. It couldn't be more true, but bon courage to you if you're taking the plunge! Even if you're just going for a visit, these items will help make your visit easier.