There's a nice group at Hastings Museum in Bohemia Rd thursdays 1-2.30. Smarties at St Matthews silverhill wed 10-11.30 but there is a waiting list. If you pick up a 'What shall we do today?' book from any childrens centre as susie says, everything will be listed.
Can't recommend anywhere around there as I am in hastings but if you google search what shall I do today hastings and you should find a pdf of a booklet with all the local groups. Failing that contact your local childrens centre and they will have a copy of the booklet. Hope this helps