Melidoni cave then Margarites then Panamo is a good day trip.
I think Plakias on the south coast is lovely. Huge beach and lots of restaurants and nice shops.
Bit further afield but also on the south coast - Loutro. It's land locked so need to drive to Chora Sfakion (aka Sfakia) and get the ferry/water taxi for a fifteen minute sail. With the number of people in your group it will probably be cheaper to get a water taxi than the ferry. Best swimming on the island in my opinion (although possibly a bit chilly in April!).
Rethymno old town is lovely. A couple of years ago there was a great restaurant called Le petit fleur on the harbour front.
Restaurant called Romeo and Juliet in a village just inland of Rethymno (can't remember the name of the village) is great, fantastic food. Was there just a couple of weeks ago, best panko prawns I have ever had.
Small tourist town of Georgiopolis west of Rethymno is nice. Traditional seaside town with just the right edge of touristy.
Beware the sea between Rethymno and georgiopolis though - some nasty rip tides. Usually a drowning every year unfortunately.
Lappa is supposed to be a must see. Aygroupolis has some interesting springs and traditional restaurants.
Second spinalonga but not sure the younger members of your group will get much out of it.
The cretans are very proud of Knossos but I preferred the archaeological museum.
The imbros gorge is an easy walk but again probably not fir the young ones.