That is a really hard question as depends on so many things.
I love to just vegg out on weekends, but I like doing this all the time.
If I had a partner, I used to like going out to the Hills for lunch -there are a few really good cafes including an excellent vegetarian one.
Weekends are for switching off - grab a book, a blanket and some snacks and head to a spot with a fantastic view. Anywhere along the Swan River, or the gardens of Kings Park. While you're there, wander around and have a look at the beautiful boab tree.