We have fun on our bikes every day of the week, while we use them for transportation. The only difference is on the weekend we don't usually have to check in at work. I know that after I sold the car my weekend rides just to ride became rare. My description of last weekend- get some things done along the way extend the errand running is more typical. Sometimes I take camping trips or what not or explore a new place. I also do long fun rides if I'm getting in shape for a tour.
I find myself doing more and more "fun" transportational rides on Saturday ... trip to the library, health food store, LBS, even Walmart once in a while. Then Sundays, I like to head out of town on a purely recreational run. If you set all the transportation rides up nicely (not too much shopping, for example....), they can be a lot of fun.
My only problem with the Sunday ride is that get to the half-way point of the trip and realize I should turn around. I have this tremendous urge to keep going....