Well, if you like the photos you should go for a pub crawl in a part of Helsinki called Kallio. The beer is MUCH cheaper than in the other parts (Pub Roskapankki 2 euros/pint). If you’d like to check out the club scene I’d recommend the neighbourhoods of Kamppi and Punavuoret. There are a couple of nice ones in Kallio as well actually. To check out some club venues you should look up this page: http://www.klubitus.org/kalenteri.php
If you’d like to check out a concert you could go to the Tavastia Rock Club: http://www.tavastiaklubi.fi/tavastia.php?lang=en
Art freaks will enjoy Kiasma, the museum for contemporary art: http://www.kiasma.fi
If you’re in town already on the 24:th of August you are in luck as the whole city celebrates the Night of the Arts – lots of free happenings throughout the city.
Kafe Mockba (http://www.andorra.fi/moskova/index.html) is a surreal place to sit for a drink. It’s partly owned by director Aki Kaurismäki (http://www.imdb.com/name/nm0442454/) who likes his own style (more or less eastern european nostalgia). There’s some bigger establishments in the same city block in conjuction with Mockba called Dubrovnik, Corona and a movie theater called Andorra.
Nice nature spots can be found in Haltiala at the Pitkäkoski rapids (good place to head off to with your rented bike. You can cycle through woods almost the whole way), a spot called Kustaanmiekka in Suomenlinna (from there you can check out the big ferrys that come into Helsinki), take a walk in Kaivopuisto park in southern Helsinki, a classic restaurant is Sea Horse (in the Ullanlinna neighbourhood; http://www.seahorse.fi/indexsea.html) that serves simple but good food. Ville Valo from HIM likes to eat there.
There you have some tips. Hope it helps. But keep your options open. You’ll have the most fun if you take some chances on your own.