with regards to the weather, the rainy season is approximately nov-jan while the hottest months are about june to sep. depending on where you plan to go, the weathe may or may not be a big consideration - in singapore, where i come from, many/most places are air-conditioned, so the heat is not a huge putoff. also i guess you'd have to consider when the festivites are, eg. the chinese and islamic new year festivals, loy krathong in thailand which is absolutely beautiful and romantic, massive sales and a food festival in singapore in june etc. not too sure about the countries further north, eg. thailand, vietnam, laos, myanmar and cambodia. diving is another thing you might want to do, i believe west malaysia (borneo), manado in indonesia and other places are phenomenal too.