Grover Beach offers fun things to do, many of them free.

The Oceano Dunes State Vehicular Park offers the only driveable beach in the state of California. Rent or bring your ATV and kick up some sand. (You can bring your own truck, car or motorcycle but be forewarned - if you get stuck, you will have to pay to get it towed out.) 

Grover Beach also offers a great beach for strolling and playng in the water. Easiest place to enter is at the end of Grand Avenue.

The Monarch Butterfly Park, on the border with Grover Beach is interesting (and free) year round, but is most interesting from December - February, when thousands of Monarch Butterflies migrate here to spend the winter is a warm climate. The trees are covered with them, and there is a free docent talk on most days to explain about this wonder of nature.

The nearby tiny town of Oceano offers a big attraction - The Great American Melodrama. For over 33 years, they have offered professional year-round family entertainment, except for the first 3 weeks in January. The "Melodrama" change their play many times a year, and in summer alternate between two different plays. Each play is either a comedy or melodrama, giving you a chance to boo the villain and cheer the hero and heroine. Each performance is paired with a fun and different vaudeville review. The actors show you to your seat and also serve you in the snack bar, which offers chips, hotdogs, popcorn, beer, wine soda etc. They do ask the audience if anyone there has a recent birthday or anniversary, and then the audience sings Happy Birthday/Anniversary, so this is popular for people celebrating those events. Reservations are advised, especially during their extremely popular Christmas show (mid November - Dec 31.) 

The town also offers a 9-hole golf course, right next to the ocean.