Regarding NYE fireworks, there are usually two main sites of fireworks, one at Central World near Siam which usually gets in excess of 1 million people and the other by the river between Saphan Taksin and Memorial bridge in which case staying at one of the river hotels north of ST is great. (Note that Rama 8 bridge may also have a show as there are also usually some fireworks in this area).

The advice of staying at a tall hotel may work if you want to see both shows but usually the river one is more scenic though the CentralWorld one is more popular. Have to wait for the BMA to announce exactly what will be happening this year if they have not already done so.

In the last 10 years the Central World show has been cancelled once, or twice, due to security fears but as long as nothing happens in the period preceding NYs then that is unlikely.