Our DS is also 5 and not accelerated at school. This is what he likes at the moment:
- going to the zoo and asking questions about every animal
- reading Click magazine
- Lego. Building, improving, rebuilding... He's even happy just reading the instructions 😁
- reading: current favourite in English (his second language) is Roald Dahl
- going to the reading group at our local library (which is free and actually aimed at 7-year-olds, we have a wonderful librarian)
- science museum
- riding his bike
- playing board games. One of our favourite is Magic Maze. We always prefer 'regular' versions above junior. If necessary, you can adapt the rules in the beginning (not often necessary in our family) and then you'll have a game you can play for years.
- helping around the house and discussing how everything works

I'm sure you're doing great just being there for your son and encouraging his interests!