A few miles east of Kenilworth, Stoneleigh Abbey is a country house grown out of a Cistercian abbey founded in 1154. After the Dissolution it passed to the Leigh family, who incorporated the medieval gatehouse into a Palladian mansion. Jane Austen visited in 1806; her mother was a Leigh, and elements of the estate are widely thought to have informed the descriptions of Mansfield Park.
The house is open seasonally for guided tours, and the grounds — landscaped by Humphry Repton — are free to wander on event days. A worthwhile detour from Kenilworth Castle if you are spending the day on local history.