Goa is a 365 days holiday destination. but I prefer to avoid April and May due to heat. though I had visited Goa in last April. daytime on beach has been quite hot. you can visit North Goa beaches, church, famous clubs. water sports, scuba is to do activities. South Goa and Dudhsagar fall is also enjoyable.
Since it's very hot now, if you guys have not visited do experience Goa in the monsoon. it's off season but amazing to experience in that season. try checking out the other side of Goa i.e. Palolem / Cola Beach side.
You can visit now . It will be pleasant. Or you can go anytime after September.
Monsoons are also very good time to go Goa . however that time sea is rough so you can not enjoy water sports
Goa can be planned any time except summer. You can include activities like visiting churches, beaches, water sports, tasting local cuisine including beer, wine and fenny, going on night cruise.
Personally, I prefer visiting Goa between September and March. Because, after April goa starts to heat up and the weather is not so pleasant to enjoy your vacations much.
It think the best time will be from November to March
but if you want to enjoy completely so I recommend you to go in December near occasion of Christmas or new year.
You can visit goa till the end of march. Avoid the months of april and may. There's lot to do in goa. Beaches , Water sports , Clubs and Pubs , Night Market , a couple of museums , churches and forts.
Sepp march is recommende but i travelled once in june. , you can see goa monsoon side then less crowded and less rates for everything. so plan in advance.
If you want crowd then nov, dec and jan is best and rest you can visit in feb and march also but dont go between april to October as it gets realy hot there...