Magnolia can get pretty busy but it wont be anything like South Padre. If you want a covered pavilion I recommend getting there before 10am. The busiest location will be just before Indian Point in front of Old Town Lake on N. Ocean Dr. so you should probably avoid that area. Pass up the point and you should be able to find a spot to spend the day.
If you plan on fishing there are some submerged rocks close to the beach area
here is a map for you that hold reds & trout most of the year (I fish it with artificial). Just be careful, the rocks are sharp and falling will probably result in plenty of cuts.
Have fun.