Formal:
* Prettig weekend! (pronounced: "preh-tihg wee-kend") - This is the most common and polite way to say it.
* Geniet van uw weekend! (pronounced: "guh-neet van oo wee-kend") - This means "enjoy your weekend!" and is also very polite.
Informal:
* Fijn weekend! (pronounced: "fayn wee-kend") - This is a more casual way to say it, suitable for friends and family.
* Goed weekend! (pronounced: "good wee-kend") - This is a very informal way to say it, suitable for close friends.
You can also use the Dutch phrase "Fijn weekend!" which is understood in Flemish.
Ultimately, the best way to say "have a nice weekend" in Flemish depends on the context and your relationship with the person you're speaking to.