Lena,
You could say...
Bon appetit.
Good appetite.
Enjoy your meal.
Enjoy your dinner, breakfast, supper, etc.
Have a good meal.
I hope you will find everything to your taste.
Savor your meal.
Regale your tastebuds.
If the meal is informal...
Eat hearty.
Eat up.
Chow down.
Dig in.
Gorge yourself.
Tuck in.
Pig out.
Stuff yourself.