Yes, that's way too much for 8 days. You don't want to go to Monteverde for just one day. It's a pain to get there, and you won't want to go there and come right back. The Arenal area has a lot to see and do. Most people stay at least three nights there. Silencio del Campo is a popular place to stay if you want to explore that. I'd do a split between Arenal and one beach destination and your time required in San Jose for your flights, otherwise it's too much moving around.

I agree, Monteverde or Arenal, but not both. If you are talking about day trips from Tamarindo (I would recommend Playa Flamingo or Hermosa, or even Conchal in that area over Tamarindo, but that's personal preference) or Guanacaste, Rincon is fine, less than an hour from Liberia, but the others are not "day" trips.
It's 4 hours from SJO to Tamarindo. Here's a driving time chart (don't get your Playa Hermosas confused): http://www.govisitcostarica.com/travelInfo/travelTime.asp#24
Here are some inexpensive suggestions, no idea of availability:
Playa Flamingo/Brasilito/Conchal, less than $100: http://www.lacarolinacondo.com/
Arenal: volcano observatory lodge, starts at <$100:
http://www.arenalobservatorylodge.com/en/rates-a-reservations.php
Monteverde (terrific deal): http://www.lospinos.net/

I have my own biases and preferences, but I wouldn't bother treking down to Manual Antonio (been there low season and high season, and found it crowded, and logistics for a day at the beach with parking, park fees, not worth it. JMO! No suggestions for lodging there; 2 places we've stayed at are booked.)
Ditto, ARenal can be very expensive; tours, hot springs (especially), zip lines all fun but pricey. Monteverde is a bit less developed tourist-wise, but note the time chart for the drive, and you WILL want at least a small 4WD.

Check this forum for car rental suggestions.

Thank you all for your recommendations! I will check out the recommended hotels.

Agree with everyone above. Driving to Tamarindo via Nosara is possible in a day, but long and bumpy, and I'd recommend spending a night or so in Nosara, or driving direct to Tamarindo, and then make your beachcombing excursions from there.

Yes, you could do a day trip to Rincon de la Vieja, but it might also be great to spend the night.

Other great day trips include El Viejo Wetlands, a range of canopy tours, the Llano de Cortes waterfall, snorkeling trips.

After that, yes, either Arenal or Monteverde, but not both.

And for beach either Manuel Antonio or Guanacaste (Tamarindo/Nosara and all the rest), but not both.

Availability is tight so close to the busiest couple of weeks of the year, I know, I'm looking around for clients.

BTW, here's a different site for La Carolina (owners have various websites); not sure if they updated availability, but price and location are pretty good (walk to restaurants, a few miles from Flamingo, a short drive over the beach to Playa Conchal): http://www.flipkey.com/playa-flamingo-vacation-rentals/p149281/

Agreed with everybody here that your itinerary is too rushed for eight days.

One other point I'll add is that some lodgings impose minimum lengths of stay for the week between Christmas and New Year.

And wow it's a pricey time to go. Prices really go up around December 15th or so. Hope you find some good spots you like.

Roy is an advertising troll promoting a personal business.