In Otwock make sure to take a stroll and look for Świdermajer-style houses:
https://en.wikipedia.org/wiki/%C5%9Awidermajer
If you'd like to wander a little bit I'd recommend Milanówek with plenty od 1920's and 1930's style villas. Interesting choice might be also Żyrardów and its industrial part with lofts and interestingly arranged space.