We're going to Charleston the first weekend in April, then to HHI the following weekend (wedding season!). Rather than flying back and forth to New England, we're thinking of just staying for the week in-between - perhaps Savannah? Is there enough there to do/see for 5-6 days? Or should we mix it up with some beach time somewhere? Appreciate any thoughts/suggestions - thanks!

Beaufort, SC is an historic town with lovely antebelleum homes and trees dripping with Spanish moss and a fairly lively downtown. There is the Spanish Moss Trail, a rail-trail for bicyclists and walkers, which goes to Yemasee. But maybe only two nights/three days worth of things to see/do.

Savannah is a lovely city with many different things to do: River Street, charming neighborhoods, arts and culture, City Market, Bonaventure Cemetery, historical sites, shopping, and much more. If you want beach time, Tybee Island is right there.

I just reokued what to do in Savannah in a separate thread so that would be my recommendation. You could also visit the beach at Tybee Island which is nearby. I also like Beaufort but do not think there would be enough for 5-6 days. You could split your time between the three options I mentioned. You could stay in Savannah and just drive to Tybee Island for the day.

I'm not totally clear on how long you will be in charleston and Hilton Head. There is plenty to do and you might want to go back to Charleston for a few days if you only spent the weekend. I guess you will be in HH for a wedding for the weekend--that's plenty of time there.
The post about Savannah certainly would give you several days there. And with the mention of Beaufort, read Midnight in the Garden of Good and Evil and see that link to Savannah!!
Plenty of beaches from Wild Dunes outside Charleston to the others mentioned--Edisto is an old one.
It will be garden tour time everywhere along the coast with the azaleas and dogwoods in bloom. Plantation visits would be beautiful.