I have travelled extensively with toddlers as we are a pretty nomadic family. I’m not one of these types who refuse to go on any abroad holiday just because they have an under 5- but quite honestly I have found city breaks merely ‘fine’.
tbh the most enjoyable bits have been the nice dinners when they have been sleeping. Kids museums and aquariums aren’t usually how I want to spend my time when on holiday, and I had a toddler who was extremely energetic and hated staying in the pram. A lot of running around at a 90 degree angle keeping them from tourist tat.
if you want to go anywhere- then Copenhagen, Stockholm and Amsterdam are probably your best bets. Or one of the big Swiss cities. Lots of kid friendly things, parks and a cool climate. Public transport is generally accessible to.
In southern Europe you will absolutely have the standard family museums but generally you are restricted to usually (hot) parks for giving them the run around unless you are lucky enough to be near a soft play or kids museum to spend an hour before nap time. have found Italy and Croatia particularly bad for entertaining children of this age group (travelled both places extensively- happy to discuss further in a PM). Croatia especially because all the toddlers are in full time nursery because it’s free so you have zero infrastructure for kids.
In the aforementioned scandi and Swiss places, there is always a bloody ‘culture house’ with a soft play area a stone’s throw away, random organic stone shapes for kids to climb on and Tripp trapp high chairs everywhere if you want to stop for a coffee.
City breaks are best when you have an 8 month old potato who is happy in a sling, or an older child who can actively engage in the exhibits at whatever museum you are in.
I usually tell people to take the trip- but I do think centerparcs, big air BnB with pool and other friends to help or club med with mini club is best for this age group.
You will all get so much more out of it if you wait another year and a half. The difference between a 3 year old in attention span, stamina and engagement compared to an 18 month old is night and day