Actually I am going for 兩日一夜 too. It's $169 (before tax) for a night including breakfast for me. I think it's cheaper in March and summer seems to be high season for Harrison (cos of water sports?)
i checked in at 3 p.m. last time. No problem at all. (btw they serve cookies and tea at the main lobby at 4 p.m. so get yourself a free snack before heading down the pool.
as far as I remember, check out time is ... 11 a.m.