I also live in the West Midlands and to be honest the whole area is surrounded by excellent countryside.
It may make a difference which side of Birmingham you live (north, south, east, west) as it is obvioulsly easier to travel to the countryside nearest to you than to travel across the city first.
If you live in the south or south west then you have the Malverns, Worcester, Gloucester, Cheltenham, Evesham and all that area.
In you live in the South East then the Warwickshire countryside with Stratford on Avon and the Cotswolds.
If you live on the East then East Anglia is a beautiful unspolit area (in fact the north norfolk coast is one of the nicest areas in England). How about Hunstanton.
If you live in the North or North east then the Peak District is on your doorstep, for example Matlock.
And in the North West you have Shrewsbury and surrounding areas, even Wales is not too far.
Sorry to give you so many choices, I have probably made your decision harder.