Hola, haven't been online much the last week or so. Regarding hot springs: There's a whole bunch close to La Fortuna/Arenal volcano. The springs is new and luxurious, Tabacon the most beautiful and most popular. There's a whole bunch more. All of them are nice,especially after a day of riding!
San Jose - Tamarindo will take longer than 4 hours, probably closer to 6. Flying into Liberia is much more convenient when you're planning to go to Tamarindo.
On the Panamericana and other bigger roads always obey speed limits. Even if you do so they might stop you in your obvious rental car and make up stuff so you'll pay them something. If they ask for $150 or whatever other amount, just tell them you only have $20 or $40 with you. Such an amount normally does the trick. Make sure you don't have hundreds of $$ in your wallet when they ask for your license...
If anyone is planning to visit the Arenal area and would like to trade a set for bringing down some stuff let me know! Always looking for people to bring goods from the US.