Nearby Islands...
One of the better things about being on an island like Crete is that getting off it for a day or more is relatively easy, and if you happen to be on holiday then you can add a second island to your trophy room and be back before the day is out.  There are several small islands off NW Crete.  Some, like Gramvousa you can reach as part of a driving or hiking day out.  But the easiest and most magical way for the non-specialist holiday-maker is by boat.  And the kids will love you for it!
Motor Vessel Gramvousa Express Kastelli Harbour. Gramvousa, Balos, Venetian Fort. Gramvousa and Balos...
This is the Gramvousa Express, at berth in the harbour of Kastelli.
In the summer it makes a daily run to Gramvousa, where sun-lovers and swimmers have 3 hours to enjoy the stop, and then proceeds to Balos ('Valos') about ten minutes away for another three hour or so sojourn, retuning to Kastelli at 6pm.  The fares work out at a little less than 11 GB Pounds per person for adults and about 5 GB Pounds for children.  Thirty pounds or so for a family of four for the day.
At the beginning of July the Gramvousa Express Started from Kastelli at 09.30am Daily.  Arrived Gramvousa at around 10.30am.  Departed Gramvousa at around 1.30pm.  Arrived Balos about 1.40pm (ten minutes later).  Departed Balos about 4.30pm.  Arrived Kastelli at about 6pm.  The easiest way to get to Kastelli harbour, if you are not part of an organised group, is by car or taxi.
Sitting off the headland...
Sitting off the headland of the Gramvousa Peninsula, the somewhat flat-topped tiny island  of Gramvousa and its neigbouring and even even smaller jutting mass of Balos rise from a fairy tale green-blue sea in summer.  Gramvousa on the landward side having very shallow bays and an isthmus of golden sand reaching out from the headland.  The Gramvousa ferry actually lands passengers at a jetty below an old Venetian fort, now in a state of some ruin.  There are two things to do.  Visit the fort or swim!  If you choose the latter take care on the climb up and down, and beware pot-holes.  If you want to picnic remember your cool box.

Later at Balos Lagoon - you have to transfer to the Island by a smaller boat - you will have the opportunity to swim in the beautiful waters, or if like photography get some really outstanding pictures from a spot not exactly overrun with branches of Jessops!!  In fact toilets are in short supply too.  Be warned.  There are no tourist facilities and the sun can be very, very hot.  Plenty of sunscreen is a must here.  Having said that two of our guests felt that the day was too long and hot, but at the same time was well worth doing.  It is.
Kithira is a small island sitting to the north and slightly west of Kastelli.  It is about 4 hours sailing time from Kastelli and about three hours from Githio on the mainland Peloponnese.  I have never done this trip but it is one that I intend to this year as a photographic outing.  Taking into account current sailing times of the ANEK car ferry Myrtidiotissa from Kastelli, it is possible to catch the 0.8am sailing from Kastelli and arrive in Kithira at 12noon local time.  The return sailing leaves at 8pm and arrives back in Kastelli at midnight.  8 hours on Kithira.  It is of course possible to seek other sailings on different days  - mine is based on one day.

A brief outline of these sailings is given on the ferryboats page.
All my suggestions here are based on the ANEK Lines published sailing times in July 2001.  Anyone contemplating the same trip, especially if you are on holiday and working to a deadline would be well advised to consult ANEK or a travel agent such as Paleologos in Iraklion (Link at on the home page) who can confirm latest itineries and accept and confirm bookings online.