Some of the big attractions of Havasu are the swimming holes. February might be a bit chilly for swimming, but on the flip side, it would be cooler hiking out. The crowds should be a little lighter than, say, Memorial Day, too.
I'd recommend two nights at the campground, with a dayhike (+/- 12 mi. R/T) to the Colorado River on the layover day.
Either way, remember to stop in downtown (?) Seligman @ DelGadillo's Sno-Cap. It is not to be missed.... an Arizona icon.