I wouldn't be able to cycle though!
It would only take you a couple of weeks maybe ;)
There is another lake that I would love to visit, but it is in the Tatra National Park, so unfortunately not close to Poznań.
I have seen friends' pictures of this lake and it has beautiful clear water surrounded by stunning scenery.
This is it:-
en.wikipedia.org/wiki/Morskie_Oko
You never know, maybe you will get a chance to go with friends or something, but it is a bit of a long trip from where you are.