There is a double-decker London Bus (1969) doing a two hour tour around Helsinki on Vappu morning. Very informal I have been told. The tour is arranged by Finnbrits in Helsinki but it is open to all. ( Yes, I am sure ! ) The bus leaves from the Swedish Theatre at 10.15 am, but be there at 10.00 to be sure. Price 7 Euros, 4 Euros for kids. It is not not so much a tour as a trip explaining how Helsinki tour guides present Helsinki to foreign visitors.
There´ll be a second tour at 12.35 if a lot of people turn up and one bus is not big enough for all interested.