The Seacoast of New Hampshire is an hour drive from 3 airports; Boston (BOS), Portland (PWM), and Manchester (MHT). Each airport has rental car services, however, check with each airport for additional options. If you are coming from Boston, you will have more choices available to you such as C&J Bus & Amtrak.
Finally, if you are looking for things to do, always check the various travel sites, your social media, etc. and also check out the Live Free Guide.