Swimming!
Me and my SS (he's just turned 9), have a "day of fun" a few times a year, where we do something just us. Not his Dad or sibling. We pick something fun and special and have a money box we save up to afford it. It could be pottery painting, cinema, soft play when he was younger, going out for cake. Once we saved up for Legoland! If you're looking for something more regular, driving range went down well. We had both never done it before. Other ideas: Swimming, trampoline park, bowling
Soft play, cinema, bowling. Can you google anything activity wise that is local to you