Observatories with public access in Europe
There are plenty of possibilities - and often, even when not publicly advertized, many observatories will be very open to find an arrangement when approached, especially by a group of interested audience. So this list IMHO would be the complete list of possible observatories found in central Europe.
The wiki page on astronomical outreach resources in Europe has such list of observatories (well, for the whole world, but filter it) - mostly the professional ones; of course it's not complete, e.g. the Thuringian observatory at Tautenburg is missing (who also offer regularily public tours) as does the GEO600, the gravitational wave observatory near Hannover.
There are various lists for amateur associations, many have their own observatory, e.g. this one lists many in the DACH countries sorted by post code.