Finland, Sweden, Norway, Iceland, can all work well though in Scandinavia its best to check where activities are in advance and book accommodation after that as things can be far apart. Norway esp round fjords is stunningly beautiful though very pricey. Finland and Sweden we found cheaper - preferred Finland to Sweden. When we did Finland in summer we did brown bears in the wild staying at Arola Bears where also get to canoe (previously could go to Russia too but maybe skip that one 
), you stay in a cabin with sauna by river, also did whitewater rafting, beach at Oulu, summer toboggan. Bears are seasonal so need to get right dates.
Sweden Stockholm is nice and plenty to do. Copenhagen also good though maybe more for few days than whole holiday but easy to move between places if hire car etc. Norway we did around Alesund / fjords and Flam, all very scenic and enough activities for teens but prices can be sky high for food and hotels. Weather can be mixed in Scandinavia but you avoid the strong highs and mostly it was sunny and low 20s on our trips though earlier in school holidays we got better weather than when we went mid to late August. Also their schools are different holidays so some things were closing at different times but not a huge issue.
Austria in the mountains would be cooler though on the ground temps can get high. We did Hintertux which has snow all year on their glacier, an ice palace, all year snow and ski lifts, good for few days in combination with something else, whitewater rafting in summer.
Otherwise I do water based holidays so ones with swimming pools, waterparks, accessible sea anywhere can live in the water as I am heat intolerant too so is my DS.
Outside Europe Costa Rica was amazing for teens and not too bad temp wise in August as its rainy season so about 27C but rain in rainforest after about 4pm each day but that is fun. Great for teens with high ziplines, wildlife, whitewater rafting, sea, watersports, volanoes and rainforest.