2 browsers, 1 with nos2x or nos2x-fox, the other without. Both with ad blockers, or just make Brave one of them, even though it's also not actually the best for privacy/security.

Open source versions of any tools they might use, like libreoffice, or LMMS+Audacity if they're into making music, or kdenlive if they might try starting a YouTube channel or something.