One of the first big map projects I’d love to do is combine old, hand-drawn maps of cities with 3D topographical info of the region. I remember some studios using GIS back at school…as big of a pain as working in the software was, now I wish I’d actually attempted it!
I don’t think this site gives topographical data, but for pretty much anything else (including buildings and maybe even building heights if it’s a major enough region!) there is Cadmapper:
Downloadable files in a variety of formats are free up to 1 km squared! There’s no limit on how many freebies you download as well, so with a little planning you can grab a pretty big chunk of terrain in smaller clusters. You can even specify how much or how little of the available data you would like.