We stayed at HHI for a few days in April 08. It was probably in the 60s-70 temperature wise. The resort has activities scheduled that can give you plenty to do. We were not there long enough to venture out on our own. We are going to spend Springbreak 2010 at HHI and are looking forward to it.
We've been during March several times. Weather can be iffy then, but it can also be very nice - warm days/cool nights. Activites are beginning to kick back up island-wide at that time, and the beautiful flowers begin to bloom. Late March to mid-April and October are my favorite times on HHI.
dh and I have been first week of March & daytime was high 60's/low 70's, evenings we just needed a light jacket, although we knew there could've been a cold snap but we're from the northeast so we were traveling with heavier coats, sweaters anyways too. We thought weather was perfect, enjoyed those temps more so than when we've been there in late April, May and summer months.