A visit to the cinema in Marmaris is a good way to entertain children. They run English films with Turkish subtitles. The cost per adult in July 2009 was 9 lira. There are several timings through the day. The cinema was also air-conditioned.
There seem to be 2 water parks: one in the hills and one on the water front in Marmaris. It is possible to print off a money off voucher from the internet before leaving home.
Some of the English bars will show films (when there is no football on!!).