These cities offer discount cards of sightseeing - public transport, entry fees etc which you might find useful for a city stay.

AARHUS - Denmark - http://www.visitaarhus.com
AMSTERDAM - Netherlands - http://www.amsterdametourist.nl
BARCELONA - Spain - http://www.barcelonatourisme.com
BERGEN - Norway - http://www.visitBergen.com
BRATISLAVA - Slovakia - http://www.wisitbratislava.eu
BRUSSELS - Belgium - http://www.brusselsinternational.be
BUDAPEST - Hungary - http://www.budapestinfo.hu
COPENHAGEN - Denmark - wwwcphcard.com
CORDOBA - Spain - http://www.turismodecordoba.org
DIJON - France - http://www.dijon-tourism.com
DUBLIN - Eire - http://www.dublinpass.ie
DUBROVNIK - Croatia - http://www.tzdubrovnik.hr
EDINBURGH - Scotland - http://www.edinburghpass.com
GIJON - Spain - http://www.gifjon.info
GOTHENBERG - Sweden - http://www.goteborg.com
INNSBRUCK - Austria - http://www.innsbruck.info
KRAKOW - Poland - http://www.krakow.com
LISBON - Portugal - http://www.askmelisboa.com
LONDON - England - http://www.londonpass.com
NUREMBERG - Germany - http://www.tourismus.nuernberg.de
OLOMOUC - Czech Republic - http://www.olomoucregioncard.cz
OSLO - Norway - http://www.visitoslo.com
PRAGUE - Czech Republic - http://www.praguecitycard.com
REYKJAVIK - Iceland - http://www.visitreykjavik.is
RIJEKA - Croatia - http://www.tz-rejeka.hr
SAN SEBASTIAN - Spain - http://www.sansebastianturismo.com
SPLIT - Croatia - http://www.visitsplit.com
STOCKHOLM - Sweden - http://www.stockholmtown.com
TARRAGONA - Spain - http://www.tarragonaturisme.cat
TALLIN - Estonia - http://www.tallinncard.ee
TURIN + PIEDMONT - Italy - http://www.turismotorino.org
TURKU - Finland - http://www.turkutouring.fr
VALENCIA - Spain - http://www.turisvalencia.es
ZAGREB - Croatia - http://www.zagreb-touristinfo.hr

Please feel free to add any that I haven't !!