Still not sure where you're staying. Night Heron Rd. could be a hike, not familiar with Night Heron that looks like a timeshare (different if you were staying on Kiawah though!). Anyway, the folding, lightweight beach chairs at Wal-Mart are about $10-12. You can get an umbrella there too. The tough part is securing it into the sand so it doesn't blow away....one option is a corkscrew thing you can buy that the umbrella slips in to. Altogether you've stll only spent $50.
Rolling coolers sound good but once you hit the dunes/ soft sand you will be lugging it a ways. I'd consider getting a soft sided cooler you can sling over your shoulder. You can always leave your things on the beach, go home for lunch, then come back. Just be sure to pay attention to the tide....it moves fast and covers a lot of ground and you don't want to return to your things washed away.
If you drive to the main beach access (Beach Club) you can park (get there early or go late in the day to park!) and have access to food and beverages there. There are also lifeguards stationed on this part of the beach.
When I was pg. we did the pool in the morning, then I stayed in during the hot part of the day and went to the beach in late afternoon. Much more pleasant as the sun is setting and not so brutally hot.
Oh, of course you'll use sunscreen, but I have to say, I laugh every summer as the days go by. Every Sat. the renters arrive, Sunday they spend at the beach, and on Monday you see people walking around Harbour Town with lots of sunburns! It takes 2 or 3 days of recovery before they can return to the beach. So, think about taking it easy in the sun the first couple of days. (Thursdays are nice because by then people know how to navigate the Sea Pines circle. Then the cycle starts over.)