When you are going to the city centre with the train, you can choose a ticket which works the whole day in the city (I guess some specialists here can help you out). That ticket is great if you want to explore Helsinki since you can use a busses, trams and even the ferry to Suomenlinna, this is a island near the city centre and nice to visit. Also don t miss Vanha kauppahalli (old market hall) - itīs a tourist place - yes - but you can have nice lunch in a good atmosphere (if not too many asian tourists take their pictures). Since Helsinki is pretty small you can see a lot in 8 hours.
I m not sure about the shower situation, since I not know much about the business lounge - but as far as I remember the non-schengen business lounge has limited shower facilities and you can use them on arrival and prior to departure.