i would just recommend going over the course materials and revising what you've learnt so far and what you're not so confident on. I'm in year 13 rn and thats just what i did for the easter holidays last year. i wouldn't spend too long on revision tho (sounds crazy but trust me lol), cos for the most recent half term holiday i literally spent all day every day revising, and i only had the evenings to rest, and by then i was soo tired lol, i was just thinking about sleep, and not actually appreciating my rest times, so by the time we'd been back at school for a week or two i felt soo exhausted, because i didnt give myself the chance to rest during the half term beak