The Sooke potholes are located in the Sooke Potholes Provincial park on the banks of the Sooke river. As expected this is located quite close to the town of Sooke, which is about 40km west of Victoria on Vancouver island. This can be done as a weekend bike tour from Vancouver, taking the ferry from Tsawwassen to Swartz bay, then cycling from Swartz bay along the Lochside trail and Galloping Goose trail to the Sooke potholes. The cycle is about 80km one-way and is fairly flat with some beautiful views.
Access & Fees
From Vancouver you need to get to Tsawwasen Ferry terminal. The 620 runs from Bridgeport skytrain station to Tsawwaseen but can only take two bikes at once and only runs every ~40mins. Another option is the taking the 601 from Bridgeport and getting off at the corner of Deltaport Way & Arthur Dr, from here Tsawwaseen ferry terminal is only a 15 minute cycle. This route is described here: 601 from Bridgeport to Tsawwassen (take Deltaport way west to the ferry terminal after getting off the bus).
If you have a large group and a car that can take bikes the best option is probably to meet at Bridgeport skytrain and use the car to take the bikes from Bridgeport to Tsawwassen while the some people take the bus. Buses are only supposed to take two bikes at a time and there will probably be other people who want to take their bike on the bus. If you do this option you will need to pay for parking at the ferry terminal. The parking is $16 for 24 hours so for a weekend trip the total is $32. More info can be found here: BC ferries parking info.
The ferry from Tsawwassen to Swartz bay is $15.50 one-way, so $31 total for the weekend per person. For one campsite the fees are: $25 for the first tent, $10 for the second tent, per night. Though there might be cheaper communal campsites that are only $10 per night. The campsites are run by "The Land Conservancy" and it's probably a good idea to call them and see what campsites they have available and book. More info on their website: TLC Sooke potholes.
Maps & GPS
The route is surprisingly straightforward. After starting at Swartz bay follow the signs for the Lochside Regional trail. Follow this for about 30km until you are in Sannich come to a fork in the road and a sign indicating Victoria is left, Sooke is right. Take the right turn to Sooke. This trail soon turns into the Galloping Goose Regional trail which you can follow all the way to the Sooke potholes campground. A detour to the town of Sooke is also an option, to stock up on food and alcohol.