Don't know where to go! Which countries recommended?

This is my first post here so I hope I got it right :)

Me and a friend are going on a 3-month trip this autumn. We are going through Russia, Mongolia and China, and will arrive in SE Asia some time in October, looking to stay 1-2 months.

The thing is, we are unsure whether to go to Singapore/Malaysia/Indonesia, or to stick to the mainland and do Thailand/Vietnam/Laos/Cambodia. Right now we're leaning towards the former. I would really like to hear what your recommendations are, and which areas within these would be good!

Taking into account:
Neither of us have been to the area before at all before.
It's a budget trip, so the cheaper the destination the better.
I like history and cultural sights (but I suppose all countries have plenty of these, though Angkor Vat would be cool).
We want to have sun and beach (but we're not divers). I know it's the monsoon season so we are expecting some rain, but is there somewhere that is more sunny?
Preferably not too touristy destination, but not unaccessible either (we're beginner travelers after all and this is our first long trip).

We need to fly out of southern China because of the visa, that's what made me think we might as well go to Malaysia, since it feels a bit unneccesary to fly from s.China to Vietnam. Also it feels like "everybody"'s going to Thailand/Vietnam these days (though that's not really a reason for us not to go...)?
I also wondered about getting around in Indonesia and Malaysia, are the ferries generally safe? If not the mainland would probably be easier. I also get the impression that Malaysia and Indonesia are more conservative than the countries on the mainland, would there be any safety concerns about two young women going to more non-touristy areas there?

Thanks for reading it all!