I wanna stay! Need help exploring HCMC options

So right now I'm enjoying Hoi An with my SO. The thing is that I start school in January but will still be getting the lovely Danish student support for the next six months. This got me thinking why not stay in Vietnam for another 2½ months.
My living costs should be fine as long as I stay below 900 USD.

But is this possible and will I be able to find something to do??
I'm on a 30 day single entry visa issued in Denmark.. How can I extend my stay for another 3 months? As far as I can tell the only option is to leave the county and re-enter getting a new visa.

Is this true?

Also I would need something to do! I study political science and have done my BA thesis in sociology. My work experience is primarily with social work and the usual student jobs waiter, bartending etc. Though my social work experience spans from project managing to the actual social work with troubled teenagers or big kids.

How easy would it be to find something semi meaning full to  do in Ho Chi Minh City?