Hot springs will be the choice of locals, meaning those are also likely to get very crowded.

If you are looking for seaside, I'm afraid you won't be able to rely on tour agents, because they mostly don't propose this option (to my knowledge) on this season.

I might be wrong.

Hong Kong, Macau, Zhuhai. It's not warm as such, but on the sunny days it is currently possible to venture outdoors in t-shirts/shorts. Check a weather page of your choice for climate info. 

South West Yunnan in Xishuangbanna. Beautiful place with a fairly decent climate and wild elephants!!. Check out when the rainy season is though, just to be sure.

  Guangxi has some descent spots. Guilin is a pretty nice city to visit and there's a real purty boat ride you can take from there down to Yangshuo (always a laugh but getting more touristy with each year) and from there you can visit (and i'd recommend spending a couple of days in) Longji, which is miles of tiered rice terraces and is otherwise known as the Dragon's Backbone Terrace. Beautiful spot and with some real quaint little villages tucked up in the hills. Got it's bars too though. Can't go wrong there.

Just arrived in Xishuangbanna yesterday and it's very nice here shorts and t-shirt weather.

Take the train from Nanning to the Vietnamese border.  It's cheap.  Apply for a Vietnamese visa ahead of time in Nanning (takes about 2 days).  Take the train from the Vietnamese border to either Hanoi or Saigon.  Great countryside.