You would probably really like the beaches of northern Guanacaste, like Tamarindo, Flamingo, and Playa Hermosa. This region of Costa Rica has some of the most beautiful beaches in the country, plus tons of excellent restaurants. Tamarindo in particular has a great restaurant scene and lots of things going on.
If you’d like more info, we have blog post about Tamarindo (https://www.twoweeksincostarica.com/tamarindo-where-paradise-meets-convenience/). We also have a chapter in our new book, Top 10 Costa Rica Itineraries, that has both one and two week itineraries for exploring this area of the country on a “lazy beach holiday” (see Guanacaste Beaches Chapter). You could easily use the one week itinerary for your last week after you’ve done some exploring. You may find some of the other chapters useful too for the road trip part of your vacation- we have chapters on Wildlife, Authentic Costa Rica, Birding, Eco-trekking, Adventure, etc. Here’s the link to the book on Amazon: http://amzn.to/1uv0Ctp.
Best of luck planning and let us know if you have any more questions.