Finland is wonderful in summer! They are currently experiencing a heatwave, this may still be the case next week!
Swimming costumes are definitely frowned upon at public saunas as they are deemed unhygienic. If you have your own private sauna then wear one if you wish so ( it is much more comfortable without one though).
When in Helsinki, I'd recommend visiting Suomenlinna www.suomenlinna.fi/en/ Which is an ancient island military base. It has a myriad of tunnels to explore, beautiful scenic spots and it's very popular with locals for picnics on sunny days. Take the public transport ferry there, not the overpriced tourist ones.
I wouldn't bother with the amusement park tbh. Your young adult DC might enjoy an evening at the Kallio neighbourhood, www.myhelsinki.fi/en/see-and-do/neighbourhoods/kallio-alppiharju-and-s%C3%B6rn%C3%A4inen/kallio
Find Santa Fe restaurant off Mikonkatu, the food is mediocre but the outdoor seating in the inner courtyard is hugely popular with locals (at least used to be!) and is great for people watching.
Mosquitoes - my DH is always eaten alive, me they tend to leave alone. No rhyme nor reason!
Alcohol - only beer and cider (so many cider flavours to choose from!) are sold in supermarkets, sprits and wine are sold in Alko, government controlled establishment. If you see one, stock up
Alko closes early on Saturdays and is shut on Sundays, don't get caught out with that one
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Supermarkets have a kilometre long candy and ice-cream isles. Try some of the crazy flavours, licorice-lemon ice-cream is my favourite 😍