I would approach doggy day care with great caution. There are some good ones out there, but there are farmer where, as long as there isn't major fighting, they don't really even notice whether dogs are getting emotionally bullied. A lot of owners don't know the difference either. The result can be a dog who was simply 'Under socialized to start with, who ends up actively reactive as a result of being thrown in with a bunch of other dogs. Another problem with "small dog day cares" is that many small dog people are not good but potty training their dogs, so there are dogs peeing and pooping everywhere. It's really hard for that not to rub off on your dog.
I'm not saying all day cares are like this... there are a few people on the forum who have had very good experiences. But be very careful.
Classes are great for getting your dog used to working AROUND other dogs while maintaining attention on you. But there is really no dog/dog socialization that goes on in classes beyond puppy kindergarten. Some training centers do have supervised play times. I think this is one of the BEST opportunities, because unlike day care, where who knows might be "supervising", or the dog park, where only (often clueless) owners are supervising, at a training center, it is likely that one of the training staff will be supervising a play group, (along with the owners) making it more likely that interactions will be appropriate.
My favorite form of socialization for my dogs is to have play time with a few other Havanese or small, similar temperament dogs, in my own back yard.