Hi Mandy, I hope you have a great time in Chile, even if it is only a brief stopover - and I'm delighted you'll finally get to see Easter Island! Unfortunately I don't really have any great Santiago tips as our time there was not quite as it should have been - just before departure our TM was informed by Travelsphere that it was a national holiday in Chile during our time there, so many of the attractions were shut!! After much frantic organizing by our TM and Travelsphere, an amended programme was put together. As far as I remember the airport was easy to navigate. We didn't see much of the city as it was like a ghost town. If you have the time and can organize it, it might be worth heading out of town and doing a wine tasting - we were meant to go to one of the major vineyards but it was shut. So we went to a small independent one instead, who were out to impress and as a result we got very generous measures!! The countryside outside Santiago is very picturesque, with views of the Andes. We did get to the viewing point at San Cristobel which was worth a visit. When in Chile you should try the empanadas (a bit like a Cornish pasty) - very tasty. They have them in Argentina as well.
I can't comment on what vaccinations you'd need for Easter Island, but I don't think there's any need for special ones for Santiago and we didn't need mozzie spray or malaria tablets - I was there in September, which should have been the beginning of spring but it wasn't, as it was quite cold and really the end of winter.
Hope this helps! Have a fantastic time!
Bob