In lots of cities I have been with the red T-shirt guides. They are nice and funny! Well, sometime you can get a guide in a bad mood, but generally it is a nice way to start getting to know a big city!!!
There are different kind of guided tours:
The free guided tour – you shall give a tip after the tour, but never more than the price they would charge for a payed one, that is what I think! Specially because there are lots of people in this tour!
Pub Crawl – where you meet people/tourists in a pub!
In each city they have some special tours like in Paris they have the Versailles Garden Tour, Montmartre District Tour, etc.
You can take the Spanish guide or the English one!
I did this tour in London, Amsterdam, Prague,… but of course when a host from Couchsurfing wanted to guide me, I loved too!
This is their blog:
http://www.neweuropetours.eu/blog/
And these are their websites:
Amsterdam – http://www.newamsterdamtours.com/
Paris – http://www.newparistours.com/
Berlin – http://www.newberlintours.com/
Brussels – http://www.newbrusselstours.com/
Jerusalem – http://www.newjerusalemtours.com/
Hamburg – http://www.newhamburgtours.com/
London – http://www.newlondon-tours.com/
Munich – http://www.newmunichtours.com/
Prague – http://www.newpraguetours.com/
Madrid – http://www.newmadrid-tours.com/
Tel Aviv – http://www.newtelavivtours.com/