I'll second the recommendation for a good vehicle for rainy season (or any season in CR for that matter!). You can get small 4x4-type SUVs for not much more than a small car, you'll be more comfortable, and there'll be lots of room for luggage, etc.

The main highways to the Pacific beaches have improved greatly over the past couple of years ... but on smaller, out-of-the-way roads washouts or landslides are always possible.

Even with the rain, it's worth making a detour after the beach to the hot springs near La Fortuna and Arenal. It makes for a nice scenic drive around the lake during the day, hot springs at night, and head to Monteverde for a day or two in the rainforest on your way back to San Jose.