Hi Vally,
There are a number of options, for a dull day why not explore some of the local sites of historic interest, Patara, Xanthos, Letoon, as in the heat of summer it can be too warm to fully appreciate them.
For a rainy day a Turkish Bath or spa massage will let them escape the weather and stresses, over the years Mrs EK has had a massage at Lykia, Atlantis and Patara Prince and said the Patara Prince massage was the best by far, I believe they now offer free transport from your accommodation also.
Other options depending on how sporty they are include water skiing etc.
Hope that helps....... worst case is an indoor Efes!