I have looked at a couple of options using MapMyRide...
Option A - via Clarendon
This route heads NORTH along the Veloway, turn off at O'Sullivans Beach Rd, Bains Rd, Piggott Range Rd, Easton Rd, Grants Gully Rd (thru Clarendon) Kangarilla Rd, Dashwood Gully Rd (Clarendon - Meadows Rd), thur Meadows (don't forget to stop at the Bakery!), Adelaide - Goolwa Rd, turn left on to Paris Creek Rd to Strathalbyn, Langhorne Creek Rd, Wellington Rd.
This is just under 100km in length and has 606m of ascent (675 of descent). It has some steep climbing in the early stages (bains rd is about 8%-9% grade).
Option B - via McLaren Vale
This route heads SOUTH on the Veloway. Follow it to the end and continue on the Sea & Vines bike path to McLaren Vale. Ride thru McLaren Vale main st and turn left on to the Kangarilla Rd, thru McLaren Flat to Kangarilla, turn right on to Dashwood Gully Rd (Clarendon - Meadows Rd), thru Meadows (don't forget to stop at the Bakery!), Adelaide - Goolwa Rd, turn left on to Paris Creek Rd to Strathalbyn, Langhorne Creek Rd, Wellington Rd.
This route is 110kms and has 520m of ascent (588m of descent). The climbs are not as steep as Bains Rd. It is a bit longer (10km) but is a much more scenic route with more bike paths.
If you wanted to add an extra challenge in, continue thru McLaren Vale to Willunga and do the Old Willunga Hill. Turn left at the top and follow the road to Meadows.
***NOTE*** I have not ridden the roads beyond Meadows so I cannot comment on what they are like, especially Paris Creek Rd. This road is a popular road with motorcyclists and is one of the drivers favourite roads in the annual Classic Adelaide Car Rally. The roads from Strath thru to Wellington should relatively easy going - flat (almost boring), possibly with tail winds, if you are lucky.
If it were me, I would take option B and if I was feeling lucky add the Willunga Hill challenge in as well.