Greek temples are interesting - I did Classical Civilisation at A-Level though, so biased.
The temples were used to house the statues of the gods, so you could find out about the different gods and what they represented - for example Athena and Zeus. There's lots of architecture inside the temples, you'd get great pictures on the internet of the statues and friezes, which were often battle scenes or told stories. The temple of Zeus at Olympia has a tableau of the twelve labours of Hercules. He could investigate these stories as well and the history behind the architecture.
Another thing your ds could do it to look at the way the temples were built, and maybe make a mini model himself. You could find out about how such huge structures were made at a time when there was obviously no big machinery - what did they use instead?
You could also look at the use of the temples. Sacrifices tended to take place outside the temple rather than inside, and there were parts of a temple where only priests could enter.
There are loads of good kids books on the subject, have a look in your local library and try this site