The 507 starts just a little east of Bobcaygeon off Hwy 36. If you are not able to get any farther east than that, then don't miss it. It's our Deal's Gap, although Southwood Road 13 has tighter twisties.
Not many twisties in the Newmarket area but if you go up Yonge to GreenLane, go east until it becomes Herald Road. It is rather scenic.
Around Barrie, go up the 400 to exit # 111 (Old Forbes) and go east along the Old Barrie Road to Hwy 11. Some fast sweepers here.
Around Orillia, head east on #12, north at Rama Road to County Road 45 (the Monk Road). You can run along the Monk Road all the way up to Kinmount where it becomes 503 then leads over to the north end of the 507 at Gooderham. To the west, you can try River Road running off Airport Road (County Road 18 ) just north of Mansfield thru Terra Nova over to #24 (Horning's Mills).
To the northwest of Orillia, you can try the Big Chute Road running out of Cold Water. From the top of that you can go up the 400 to #38 and head west over to Bala, then down 169 to Torrance where you can go south-east on Southwood Road 13. Makes a nice loop I've done in both directions several times.
Some sport bikers also like Hwy 118 running east off Hwy 11 half way between Gravenhurst and Bracebridge.
If you have time to get towards Renfrew, I think the roads just to the south west of there are the very best in Ontario. The 508, 511, 132, 41, 28, 515, 512, 513 are all terrific.
Hope this gives you and your buddies some ideas. These are among the best roads Southern Ontario has to offer. Enjoy, and keep the rubber side down.