Just got back from a glorious vacation at the JW Guanacaste last week. You will love it. The closest place to eat off property is in Tamarindo. About a 20 minute ride via taxi - $7 per person , each way - The roads are horrendous. Seriously horrendous! I'm a pretty assertive driver, but I wouldn't want to drive there. Lots of shenanigans! I would suggest Nibana as an excellent dinner option.
Most of the activities off-site that you can take advantage of, include transportation costs with the fee. So, I would say, if you are going to stay on property and relax - save the cash and take a taxi. You can even rent a driver for the day if you want to venture out further. When we went off property to zipline - at Pura Adventure (would HIGHLY recommend) the cost of getting there was included in the fee for the tour. Enjoy!