One option could be the new French lowlands summits, for your hypothetical UK 15 year old (by the way that’s “Ageist” as plenty of “older” people join the SOTA scheme every year). A UK 15 YO is often more likely to be able to go to France than a UK pensioner for example.
I made all my plans to activate a new un-activated OE/TL summit and then because I had another commitment, missed it by a week, when another (DL) activator did what I had planned - that was a disappointment.
By the way Jack - an activation only requires one contact - that’s the rule. The 4 contacts is for getting points. At least it’s not 44 contacts (albeit from multiple visits) as in the WWFF scheme!