What I have done so far with the kids is mainly speak Spanish in context throughout the day. This seems to be sinking in, because I would say that they understand me the majority of the time. Of course, you couldn't get them to admit it. They are very stubborn.
As for plans, starting this summer, I'm going to make a chart. Each activity (listening/reading a book, watching a movie, holding a conversation, etc.) will be worth a certain amount of points. At the end, if they reach their goal, they can buy a book. My older son, who is very goal-oriented, is pretty excited about this. My younger one, not so much. I'm going to see if I can set up some playdates with his friend who speaks only Spanish at home. Every time we see him, Nathan always peppers me with questions afterwards. "Mom, how do you say 'run' in Spanish?" or "Mom, what does 'juguete' mean? He is so intrigued with his friend who can understand/speak two languages.
I also plan to use Spanish even more throughout the day which means I need to immerse myself more. I'll talk about what I plan on doing in my next post.