Hyman's isn't even close to the best seafood in Charleston...no locals ever go there, b/c it's too crowded w/ the tourists...if you want great seafood, around the block is Hank's, which is much better...another great seafood place downtown is Coast Grille, at 39B John Street (near the Music Farm). Another decent seafood place is the Noisy Oyster, and if you must go to a more tourist oriented spot, go to A W Shucks or Bubba Gump, which imo, are better than Hymans. Their are some great seafood places over in Mt Pleasant as well...over there, check out Shem Creek Bar & Grille, or RBs both on Shem Creek, or just a little down the road at a great place called the Red Drum...another great seafood place is on the Isle of Palms, and it's called The Boathouse (awesome sunset views over the Intracoastal).
As for the beaches, I prefer the Isle of Palms b/c that is the only beach that has some bars right on the beach, that allow you to walk into off the beach...but Sullivan's Island is very peaceful and much less crowded...Folly Beach is cool but is crowded w/ more young people.
Not much is off the beaten path. Angel Oak over on John's Island is I guess, as it's the largest Live Oak in the state I believe...definitely check out one of the plantations...also, the area has some great County Parks...otherwise, walk the streets in the Lower Peninsula, check out Ft Sumter, the SC Aquarium, Waterfront Park, The Battery, The Market (it's not the Old Slave Market, like many people mistakenly believe), and many great old churches and houses.
You may be able to get a room for that type of rate at the Downtown Best Western and maybe a couple of other places, but I'm not sure...I think my dad when he visited me down there (My grandparents live there so I stay with them when I go down) stayed at the Ansonhouse Inn on E Bay and may have gotten a room for that...it's a cool hotel in an over 100 yr old warehouse, the rooms have the old exposed brick walls, and huge logs across the ceilings.