Agree with o-no-moto's mention of 245. 245 from Marysville to West Liberty is great.
287 south/west from near the Honda plant is a great twisty ride. It meets up with 245 near West Liberty at the Civil War-era Piatt Castles.
East Liberty to Bellefontaine on County 10 has some nice elevation and switchbacks. Just as you come into Zanesfield there is a park with an Old West fort back by the creek. And just north of Zanesfield on County 5 there is a bridge with a 30 foot waterfall underneath. You can climb under the bridge and get close to it. I always thought that would be a great swimming place.
Just to the north of there, 540 east of Bellefontaine has some nice curves and long climbs.
Lots of Covered Bridges around the Darby Creek, North Lewisburg area and there is another nice one on Thompson Rd east of Marysville, near Ostrander.
South of Plain City, Georgesville Rd/142 is nice down to West Jefferson, with some Metro Parks and a big suspension bridge on 3-mile creek on Lucas Rd. Then further south into the Battelle/Darby Creek Metro Parks area is nice, with lots of stopping places and hiking trails. Great stuff all down thru Darbydale/Harrisburg/Orient area.