We've heard the beach area fills up quickly and to arrive early. On a weekday, how "early" is early? We'd prefer not to go right when it opens at 8 am and is 50 degrees.

Be there by 10 and you will be good. If it is overcast anytime works. I've found overcast days are the best. Not a lot of people and not too hot. As long as it doesn't rain. Those days can be on the colder side and no one is there.

10:00 am is early enough to get a good spot on a weekday

If you're there by 10:00 you should be good. We've always found a spot weekday then