I love Heraklion - but remember it is a big city and not terribly picturesque!
To do - Archaeological Museum (I haven't, but I will do one day!), the fortress on the harbour, the market (buy your herbs), shopping!
To eat - snack on huge gyros near the Morosini fountain, for a more substantial mezes go to the sea front down at the end of 25 Augoustou, turn left and cross the road (carefully) - there is a whole rank of mezedepoleio (heavy on fish) - the Ippokambos (?) the first one you come to is good and cheap and of course you're beside the sea....roll on September, that's where we'll be eating on our last night!