If you haven't booked parking I'd imagine you'll struggle to park on campus for an Open Day, and there aren't many places within walking distance (you could possibly try Varsity bar which is just north of the main entrance but I imagine they'll have ways to prevent Open Dayers from using their car park - and I accept no responsibility if you try and fail!).
EDIT - The Cannon Park shopping centre *probably* will have space available for free, it's a quick walk via something of a back entrance onto campus via the Claycroft halls.
Easiest way to get from Leamington to campus is via the Stagecoach U1 bus - usually but not always bright pink double deckers. Get on at the top of the Parade outside Tesco. If you mean by driving, it's the same route as the bus - up Kenilworth Road (A452) until you get to the A46, east towards Coventry on that for one junction. Campus is signposted from the A46 onwards. Or go through Kenilworth, which is slower.