Skip the night in San Jose. Go directly to Arenal and then spend the second part of the week either in Montverde or the West coast. We did Tortuguero on the East coast, and it was amazing (Tortuga Lodge), then Arenal and Manuel Antonio -- three nights at each place. Tortuguero is hard to get to (plane, boat), but you could do three nights there and four nights in Arenal or Monteverde.
There are not a ton of animals in Arenal and Monteverde, but lots of birds. The best animal viewing is in the coastal rainforests, esp. down by the Osa Peninsula. But Arenal and Monteverde are the best places for activities like ziplining, white water rafting, horseback riding, tubing, etc.

I vastly preferred Monteverde to Arenal as a single traveler. Had awesome birding experience and good hikes. Also did a guided night hike to see the nocturnal animals.

OP Here. Yes, we are planning on driving (and yes I'm nervous about the road conditions). I forgot to mention, our other option would be to spend 2 nights at Monteverde, 2 nights at Arenal, and 2 nights at the northern coast.

Are you planning on driving? (Be warned: it's scary and road conditions are terrible, but you can get to more areas. It's a trade off and depends on what your level of confidence is.)

One week in Costa Rica in July. We have a 5 and a 9 year old. We are very active and adventurous. we are planning to spend our first night in San Jose so we can visit one of the museums there. Then we are planning to spend 3 nights on the Northern Pacific Coast (near Culebra). We are trying to decide if we should spend our other thee nights near, Monteverde, Arenal, or Manuel Antonio. We are leaning towards Arenal since it appears there is a lot to do in the area. Both our kids love animals. I'm looking for opinions and advice from those who have been there. I feel overwhelmed, there is so much to do and we only have a week. What would be our best use of time? Thanks in advance.