Hello hello or moi moi as we say in Finland 
I can't help you much with Ruisrock, sorry. I have never actually been there, in Turku itself though many times. Tried to find you some info about Ruisrock 2009, but even in Finnish pages nothing much too early as you mentioned. About accommodation contact www.turkutouring.fi and there Ruissalo camping, they probably can help you. About Finland car hire is extremely expensive as are most of the other things too like food, accommodation etc. Still I do recommend a visit. Midsummer is normally great time due to long summers nights(it's wierd to sit outside and it's still light 1am!!) About the weather it can be cold even in July, so be prepared. I guess you are from NZ so you are used to it. It also can be a glorious weather so.....
I would recommend a cruise to Sweden, they are fairly cheap, you'll get to see the beautiful archipelago and some great nightlife!! You can easily catch one from Turku.
Probably the best transportation option is a train or coach. Scooters are great if you are in a city, but can't go too far with them. Helsinki is obviously a must see; there visit the markets by the harbour, Suomenlinna (island), Helsinki Cathedral, have a picnic in Kaivopuisto and if you are interested in museums there is plenty to choose from. Great shopping in Helsinki too especially we are famous of different kinds of interior designers / designs like Marimekko, Iittala, Hackman etc.
From Helsinki I would take a day trip to Porvoo, beautiful old town. Google it, the easiest way. If you in Finland in mid July, vivit Pori Jazz, lots of concerts are free and just fantastic atmosphere ( and I don't even like jazz)
For a beautiful scenery I would travel to the Finnish Lakeland. It's hard to think all the things you should do.
If you have specific interests let me know and I'll try to help you guys.
Things you should try in Finland:
Strawberries (July) best in the world
Karelian Pasties
Rye Bread (yum!!!!)
Salmiakki and Salmiakki vodka (most of the foreigners won't like these, but there has been exceptions 
Sauna (it's a must, nearly everybody have one built in their homes)
Good luck with your plans and if you think of something more specific please let me know and I try to help you the best I can.
Hanna