I love Maui in March/April. You've picked a good trip down into the crater but carefully watch the weather reports. It's quite unpredictable and you could receive inclement weather that would cancel the hike. I'm talking near freezing, vertically blown rain. On one trip we drove up to the ridge not believing it could be so bad... it was worse, and turned back.
On my last trip, myself and two others were dropped off at the Sliding Sands trail head and hiked across to the camp sites at Paliku. They're beautiful sites, with a towering mountain wall, shrouded by morning mist. In the morning, we hiked out the Kaupo Gap down to the highway and had lunch at a cafe in Kaupo (Kaupo is the size of one cafe). Coming out The Gap is a grueling downhill hike with some very rocky sections obscured by vegetation, and some slippery, rocky roads, all of it steep. We were picked up at Kaupo and then camped at Ohe'o Gulch. The following day we drove through Hana. On my next trip I'd love to just hitch a ride from Kaupo to Ohe'o and then out. The locals are pretty supportive around there.
If you have the time, and can handle the logistics, spend your first night at Paliku, hike out the gap and then camp at Ohe'O gulch. Or, camp out at Ohe'O some other time, it's a great place.
I can provide more details if you like. I have family in Kula but not such that I can provide a hook-up (we're out of sorts with each other at the moment... family)