I don't know about group rides from that area and it is a bit early for me, but one ride you can do by yourself is to ride along the harbor north to Point Loma, get on Rosecrans and head toward Cabrillo Point. It is a there-and-back ride as opposed to a loop but there are some nice little climbs and it is fairly easy to navigate. You will also come across other cyclists.

A great ride from downtown is around the bay, but it looks like you will be on a tight schedule so you don't want to mis-time the ferry and end up very late.

Here's an online bike map: http://www.icommutesd.com/Bike/BikeMap.aspx