If you like quiet time surrounded by flowers and greenery, hang out at Bletchley Park in Bletchley. It's also full of great shops and restaurants, so you can spend the afternoon having a wander. If you like Bletchley Park, you'll love Woburn Abbey and Woburn Abbey Deer Park, both just a short drive away.
You can spend an afternoon browsing the exhibits at The National Museum of Computing during your trip to Milton Keynes. Check out the shops and top-notch restaurants in this family-friendly area.