You can rent bikes at the beach at this location:
SurfsideScooterRentals.com
Kayaking is available in Conway (about 20 minutes away) here:
Myrtle Beach Kayaking family fun activities in myrtle beach sc <
Brookgreen Gardens is a wonderful experience:
Brookgreen Gardens | Myrtle Beach Attractions
You can hike at Myrtle Beach State Park and fish at the pier but you will have to purchase a license. I believe you can fish from the Surfside pier as well. I'm not a fisherman so I can't tell you anything about freshwater fishing.
I personally haven't done a fishing charter but have friends who have (party boat) and had a great time.
If you like flea markets, there's a huge one on business 17 near Wild Water & Wheels. It's a fun way to spend an afternoon even if you don't buy anything.