The Baugeschichtliches Archiv ("Building History Archive") of Zurich offers a really nifty self-guided walking tour of Zurich that gives you access to some very neat historical bits of the city, including remnants of structures dating back to Roman times. It's Zurich, not Rome, of course, so what's left is fragments rather than whole structures (for a couple of the sites, all that's left are photographs of what used to be in that spot.) It's pretty curated, but it's still a cool way to spend 2 hours, and maybe fun if you've got visitors from out of town and want to show them something they wouldn't find in a guidebook.
How it works: You call up the Baugeschichtliches Archiv (044 266 86 86) to reserve a key (I have never had a problem with getting a key, even on the same day). Then you go to the Stadtsarchiv (City Archive) building in the Altstadt at Neumarkt 4 (check out the cool old model of the city while you're there), and pick up the key and a little laminated map with the sites and instructions on how to get into them. (I've always gotten the instructions in German- if you need it in English you'd have to ask whether that's an option.) Take your tour around the city, and return the key and map the following day (or on Monday if you've taken them out on Friday.)
Highlights of the tour:
Trapdoor under the Lindenhof - yup! You'd never even notice it was there, but you can use the key to open a trapdoor in the Lindenhof and go underneath it to see remnants from several eras of the Zurich city wall. This is fun for kids because there are a couple of mysterious steps you have to take to get the trapdoor to open.
Middle Ages sewer in the Altstadt - You can use the key to access a fairly long stretch of the old sewer system (obviously dry now) under the Altstadt. You walk down the passage it becomes increasingly cramped until you eventually emerge through the tiniest, narrowest old wooden door ever (crouch and turn sideways to get out!) into another part of the Altstadt where tourists and locals alike will gawk at you with "where on earth did you come from??" faces
For more info, here's the tour website (in German):
http://www.stadt-zuerich.ch/content/...e_fenster.html
Enjoy!