June signals the arrival of the south west monsoon and with it increasing rainfall largely in the form of heavy afternoon showers and lower temperatures, taking the averages back under 30°C. If you can travel in the first half of the month the chances are that you’ll arrive at one of the optimum times of the year to visit the main cities and historical sites, as conditions are very comfortable for touring as long as you don’t mind some rain interruptions.
Flights and hotels are cheaper and tourist numbers relatively low. One caveat is that most beach resorts start to close by the middle of the month in preparation for the rainy season that usually arrives by the end of the month.