We're a good of women who enjoy sightseeing, shopping, walking and good restaurants. I was thinking of trying to stay somewhere between the 2 cities, so we could do a day (or more) touring each, visiting a plantation and a garden, and also spend some time doing more relaxing things like visiting the beach. If anyone has had a good experience with a rental this size, I'd love a recommendation.
FYI, I am the unpaid, unofficial "Travel Agent" for my group of friends and family. I've planned a half dozen girls trips and tons of other trips for couples (including our famous 25th wedding anniversary trip to St. Martin with 3 other couples). One great thing about traveling with a party that size is that you can exploit economies of scale. For the same cost that you'd each pay for a regular hotel room, you can put that money towards renting a fabulous place that you couldn't have afforded on your own.
I strongly second Helen Colorado's advice. There's no place in between Savannah/HHI and Charleston worth staying. Even if you there was, you'd never be able to find a 5 bdrm rental in Podunktown, SC that would remotely compare to what you could rent in one of those tourist towns. We're talking multi-million dollar homes with million dollar views, and great restaurants and beaches just a short walk or drive away.
IMO, it makes much more sense to stay in either of these fabulous places, and just focus on all that town has to offer, instead of spending hours in a car each day?
Personally, I'd either pick Hilton Head Island or Charleston as my "primary residence." Savannah is too small, IMO. Charleston has all the amenities of a medium sized town (movie theatres, shopping malls), plus some great historical sites, such as Ft. Sumter and USS Yorktown. And, of course, there are tons of tours (walking or carriage). There's a famous open air shopping bazaar called the City Market. Nearby are a few plantations that you can tour (Magnolia is my favorite). And, of course, you have great beaches and restaurants. It's an ideal place for a girls' trip IMO, especially if you have varying degrees of fitness and energy.
Kiaweh and Seabrook Island are both great options.
HHI is just as awesome, but it's focus is more on nature. It has its incredible white sand beaches (one long beach, really) that's great for running or strolling or lounging, golf courses galore, bike trails, tennis courts, etc. You have an outstanding nature preserve just off island (Pinckney Island) or on-island (Sea Pines). If you like history, you can visit a few historical ruins, the Rose Hill Plantation just off island, or take a Gullah history tour on Dafuskie Island. You can drive (~1 hour) or take a ferry over to Savannah (in season) and either stroll around, take a trolley tour, and enjoy lunch on the wharf or Mrs. Wilkes. The Coastal Discover museum has some nice nature tours. And then there's kayaking trip through the marshes, a sunset sail, Dolphin tour on Commander Zodiac is a must (assuming you can all ride in a zodiac).
If I stayed on HHI, I'd target the Atlantic side of the island. The sound side is beautiful, but the sandy beaches are found on the Atlantic side. If budget is a concern, then forego the ocean view, no matter what place you choose.
Also, if I went to HHI, I'd only go off-season (April, May or Oct, Nov). The summer is waaaay too crowded, and the winter can be cold. Given that its biggest draw is outside activities, if it rains a ton or is cold, it can put a damper on activities.