I've heard from someone who teaches Norwegian to newly arrived expats that they simply don't get any Dutch students since these pick up the language in a matter of weeks without help. Between Dutch and German this should be even easier. Simply pick a social activity (choir, dancing, whatever) that's intended for locals, go drinking with friends, etc. If you want, of course.
I've neglected my German and French since school, but found that I could still get to a basic level of conversation later in life by just trying when I was on holiday in those countries while still living in the Netherlands. YMMV.
I lived in Sweden for a while; basically the same language as Norwegian (they share the same standardized grammar). It's easy to pick up but hard to master. Which is the same problem I have in Germany. I can get by but it's not pretty. BTW. Swedish, Norwegian, or Danish all share the same simplified grammar and are pretty easy languages to pick up if you already speak another Germanic language. Also, I did my taxes in Swedish while I was living in Finland (second language there, way easier to parse than Finnish). But I never even came close to being able to having an intelligent conversation in Swedish.
I've neglected my German and French since school, but found that I could still get to a basic level of conversation later in life by just trying when I was on holiday in those countries while still living in the Netherlands. YMMV.