If you are planning to send a day out in the nature here some ideas. I will split in 3 categories: car - bike - walk so that it makes it easier to chose what is best for you. Keep in mind that I love to walk, so what I see as a walking opportunity, you might prefer the bike or car. Everything is considered from the center of Leuven. I'll be adding more to this if opportunities come up.
Walk to Dijlepark ( 750m - 9 min one way)
Tiny park just a few min walk from the city center. The park is a perfect place to grab a book and focus on the reading.
Walk to Sint-Donatuspark (750 m - 10 min one way)
For long I knew it simply as the City Park. It's the place where you go for a walk when you feel the need to be in the nature. It's the place for summer pop up bar, for firewars and for so many other relaxing moments. And it's the most beautiful in autumn.
Walk to Keizersberg Abbey (1.2 km - 15 min one way)
The park is right outside the center. I love to go there every time I'll in the mood for a bit of a more "intense" walk as it has some stairs to reach it on one side, and a pretty damn hill on the other side. If not too many leaves..you can get a nice view over the city.
Walk to Provinciaal Domein (2.3 km - 27 min one way)
The area is perfect for everyone. Runners, people going for a walk, kids, people who enjoy a hydro-bicycle pealling on the lake, fishing ... and a lot of other nice outdoor activity,
Walk to Abdij van Park (2.3 km - 29 min one way)
Another beautiful walk which you can end with a beer at the Adij. They have cows & ponies ... CUUUTE and you have enough spots for some yoga (trust me I did go for it with teh girls. Anyway .. I walk there, if you think it's too far just grab the bike ;)
Walk to Arenberg Castle (2.6 km - 32 min one way)
This is just beautiful, I first saw it during my studies, had classes just next door. I find it especially beautify in winter covered in snow.
Cycle to Heverleebos (4.1 km - 14 min one way)
I always thought Heverleebos is far away, but then i realized I was mixing it up with Merdaalwoud (which btw I do plan to check out as well). I went there during the week and it was absolutely gorgeous. Just walk around to reach out your 10000 steps.