Is it possible for me to travel to other Schengen countries with only my passport？.
Yes, Japanese tourists can visit any given Schengen area country within their 90 days visa-free stay.
Legally* you'll have to leave Schengen zone on 15 January 2019 and within those 90 days you can travel anywhere you want.
You can also do it from Spain if you want. You don't have to travel back to Poland to legally leave the EU.* I said legally since Germany and also Spain have now more trouble than tracing one Japanese tourist who may have extended his stay. Millions of North Africans without papers are waiting there to get their "refugee status". Poland would be more strict on keeping the date.