If I were going to enrich in math, and my child was still taking math at school, I'd do problem solving, perhaps with something like
this, or with the Art of Problem Solving's Alcumus program, not necessarily kumon-style practice.
Learning a new language can be a challenge that is good for a year or two, but in an immersion program, once your child has gained some fluency, which can be much sooner than the other children might, the pace of actual instruction in the other subjects can be problematic, so if you are contemplating a placement where academic instruction will be in the target language, you might want to think carefully about this. If it is an afterschool sort of thing, and you aren't relying on it for learning other subjects, it might be a good challenge for a while, and it is certainly useful to have as many languages as you can manage to learn.