10-15 years ago, I took Ticabus from San Jose to Managua. Not a horrible trip, but long and exhausting. Even if you're just going over the border, I wouldn't do less than 2 days.
Your trip looks way too ambitious. I'd actually stay in San Jose for a day or two. They have a parade in December, and it can be interesting around the Christmas season, although use common sense tactics for valuables.
I'd do Manuel Antonio, Arenal, and somewhere in Guanacaste. December can be high season, so I'd try to book early, and look for a value proposition.