You could travel from Weymouth to Jersey or Guernsey on the Seacat for the day, it used to be Condor Ferries which ran this service, it's around 10 years now since I went for the day and the day included a quick coach tour of the island. I suggest that you check when in Weymouth or ring them in adavnce.
Severn Stars at Wool is very good - you may need to book at busy times. Day trip to Guernsy by Condor fast cat is very good as it only takes 2 hours and St Peter's Port is a good place to visit or shop in.
coljac, further to my last reply there is a ferry service from Poole to channel islands. Details can be found at www.directferries.co.uk/jersey_poole_ferry.htm
There is a very good pub called the Frampton Arms which does really good food which is at the railway level crossing / station within 2 minutes walk from the site.
Day returns from the station to Weymouth are cheap and services are regular as it is the main line service to Waterloo. The Weymouth station is within 300 metres from the beach.