What to do in last 4 days (I'm in Bialystok) - I visited Warsaw, Krakow, Gdansk, Vilnius and Tykocin already

Hi everyone,
Im in Bialystok for 1 month for an exchange. During the time, I visited warsaw, krakow, gdansk, vilnius, tykocin, now I will have 4 more days after this week to spend in Poland, waiting for my flight. Where do you recommend to go?

Criterias should be;
1- close to warsaw (preferably close to bialystok as well)
2- easy to travel to.
3- has something to see.

Thank you for your recommendations in advance.

Białowieża National Park. You can get a bus direct from Białystok. Frankly, it would be amazing if you spent time in this region and didn't visit BNP. You can visit the Strict Reserve (with a guide only, and really only for the serious student of ecology), the beautiful Palace Park, and the Bison Reserve (ask the driver to let you off; it's a couple of kilometres from town), where you can see not only bison, but also żubroń, tarpan, elk, deer, moose, wolf and lynx. After the reserve you could walk to the Place of Power (miejsce mocy), which is nearby. Alternatively, you could walk into town via Żebra żubra (bison's ribs), a raised walkway with educational signs. If you're feeling rich, have lunch at Carska restaurant. If not, the Żubrówka hotel has a good restaurant.

Roger is right. All depends what you fancy. If nature is your cup of tea, I would also head north to Suwalszczyzna for the lakes and the scenery in the Wigry National Park or Suwałki Landscape Park which is excellent for cycling, also Augustów is nice in the summer, as is the Lithuanian minority area near the Lithianian border (Puńsk/Sejny) are all worth a visit, but you would really need a car and take time to get to the right places...