I'm in the same position as you but my school gave us little head start textbooks for science and bridging work to complete over the summer. In terms of how much work to do, by no means wear yourself out or spend entire days doing it - we have such a long summer. I've got a couple of weeks in the summer when I'm not on holiday so I'm planning to do the bridging work then, when I have nothing better to do! Just try to enjoy it! I agree with the guy who suggested Memrise - I swear by it for languages, such a non boring way to learn and it really sticks, even if you did 5 words a day your Spanish vocabulary would be so much bigger by the end of the summer!