Andy -
If you use the bus, the Los Caribenos station in San Jose is not just a bare bones box with benches. There are a variety of shops, including multiple places to eat while waiting. Make sure to be in line well before departure time as it seemed to be a popular bus and may fill up, leaving standing room only. Liz and I took the bus out of Sarapiqui back to San Jose and had to stand the entire 90 minute trip.
Once the bus leaves the San Jose terminal, it doesn't stop much until the crossroads at Santa Clara where the choice is to go to Guapiles and the coast or north to Sarapiqui. There is a tienda (convenience store) there and the bus stops to give everyone a break and maybe pick up or exchange passengers. Much of the slow going is from there to Sarapiqui, when the bus stops frequently to discharge or pick up other passengers. Still, it's not that long a ride and if you've never been in the Costa Rican countryside, the scenery is interesting. You might get some stares from the locals, especially children, but they're polite glances of interest not threatening at all. The younger generations are more likely to speak English if you try to strike up a conversation.
Sarapiqui is a small, walkable town with several medium-sized grocery stores, banks, restaurants, etc. You can hear Howler Monkeys and see Macaws as well as many other tropical birds right from main street.
If you don't want to backtrack out of Sarapiqui to La Selva, you will have to convey to the driver that you would like to be dropped off at the La Selva entrance road. They seem to be very accommodating.