Prague is fairly compact . everywhere is walkable but if things get too much you can hop on a tram . We also loved salzburg which again is compact and easy to navigate round

lyn

Krakow is also fairly compact and a beautiful city or try Barcelona and use the Tourist bus to get round the sights.

lots of the streets in Prague are cobbled Molly so make sure you take flat shoes.

We're going ahead and booking a trip to Prague. It seems the ideal city for us at the moment

We did think seriously about Barcelona but the heat was another factor that put me off so we're saving that trip till after the baby!

Thanks for all your replies and advice!

In this case Vilnius could be the right choise. Beautiful and very compact

I'd add Tallin, Wroclaw, Gdansk...