I love the ease of errands by bike. I do commute part-way by bike (to the bus freeway stop) but it's only 2 miles each way so it feels wimpy. In the summer, though, I'll ride 5 to a different bus stop and a hill on the way home.

Anyway, my favorite is to bike to my girlfriend's house for our Saturday morning walk.

Second favorite is to ride to Trader Joe's and back, which follows my bus commute route but is about 4 miles each way. I have the Detours Toucan pannier, which is lovely. And a short favorite is to the local grocery store/hardware store in our neighborhood.