I would add at least a day to Monteverde. You only have 1 full day there, the rest will be driving to and from. Day #13? Are you staying near Poas? The volcano is often clouded over not long after opening, so if you're driving there from Arenal you may just see it covered. I would also add a day to PV, since it's a lot of driving for only 2 full days.

Here is a good website to see driving routes and how long it takes - http://www.yourtravelmap.com/costarica/travelmap/index.php?&width=1024

IMO best thing is to stay in Puerto Viejo area, there is so much to do there, and you will have already many days spent in the car and on the road. One full day visit Manzanillo with its Reserve, one full day visit Cahuita and hike the NP there, three days for realx and enjoying Punta Uva and other beaches.

If you enjoy hiking and wildlife, I'd take a look at the Osa Peninsula. The wildlife there is the best we've ever seen.

I agree with the others. Way better to add some extra days to your current itinerary stops rather than add another location. Monteverde and the Caribbean have enough to keep you busy for extra days. Have a fantastic time!

Someone may have covered this elsewhere. Do you get into SJO early enough to get through immigration, etc., pick up a (large) car or van and drive to Manuel Antonio before dusk? Have you planned on a car or van that will carry 5 or 6 (can't tell which) people AND their luggage *comfortably* on day-long drives? Consider making the first or last three days (of the 19) a trip to Tortuguero. For instance, plan on spending your first night in a hotel close to where you will pick up your rental vehicle. Find a Tortugero lodge that includes transportation from San Jose all the way to the lodge and back. Do the 2 or 3 day Tortuguero package and have them return you to the hotel you started from. (Plan on reducing your luggage to about 25 pounds per person, leaving the excess at your hotel.) When you return from Tortuguero, get your rental vehicle and go on with your journey. Tortuguero is sort of rustic but absolutely fascinating. This way, you would save 3 days vehicle rental.

If you really want to add another location from PV, you could go to bocas del toro, panama. You can't take a rental car there, but you can get a van/boat arranged by a travel agency in PV. It's about a 2 1/2- 3 hr trip. Beautiful area! Two days is enough time there.

I would add a night in Montevere and spend the rest in Puerto Viejo, similar to what others recommended.