flyingbee- Overnight parking (8pm-7am) was only $6 next to Travelodge Southbank. I recall it was expensive during regular hours, but the overnight hours met our return time from PI and our early departure for the airport. The lot is Wilson Park (looked at the receipt) and payment is automated using a machine that takes cash or CC. If you are elsewhere in MEL it might even be worthwhile to park there and walk or take a bus to your hotel. The city is very walkable.

Unless you're in it for the scenery, I believe taking the Portsea ferry will cut very little time off your drive vs. returning to MEL. (Assuming rush hour isn't involved). There is a lot of freeway from Geelong to MEL and out towards PI. The route along the bay is secondary roads. That's my impression, maybe a local will correct me.