Might be going to Beijing for a few weeks for work, and was wondering if its possible to do weekend trips to other countries in Asia.

I am thinking - leaving late friday night from Beijing, and then arriving back in Beijing late sunday night.

Are these places possible/easy to do, both in terms of flights and seeing a lot of tourist sites in 1.5-2days:
South Korea
Tokyo
Taiwan
Thailand

Any other countries that you might recommend? (went to Singapore and Hong Kong already last year)

Thanks