I travelled in an ordinary express bus and it was not a good one. For holidays, travelling at night will be the best. If you are travelling for business purpose then be their one day before.
Cochin is a nice place to spend holiday and itís a long journey of 16 hours.
Itís better to carry an iPod with you. There are three hotels inside this reserve; KTDC Lake Palace, KTDC Aranya Nivas and a budget hotel KTDC Periyar House. If your stay is in any of these hotels, make sure you reach hotel before 5 pm as entrance will be closed after this.