Actually, our Saint was very playful when she was younger probably from 12weeks to about 6/7 years old. We did have 2 older dogs as well at the time, which were also very playful, so I'm not sure if that influenced how she was.
For the first year we had to be careful not to over-exercise her due to her growing bones (as with all giant breeds), but she loved playing hide and seek in the woods and playing fetch.
I was originally thinking of her middle/old age where she was much calmer.
She died short before she was 11yo, and was pretty 'young' in her outlook until the last 6-8 months, although running around like a loon and playing fetch stopped around 7 yo.
She was a continental Saint Bernard though. They're more athletic looking and less wrinkly than English Saints.
She was a beautiful dog, inside and out, and 8 yrs later I still miss her.
We put a lot of effort into training her (1 on 1 lessons for her first year, as well as group lessons) and she was fantastic on the lead. Even children could hold her and she never pulled, no matter what appeared.
She never walked past your leg and the lead was always slack. Being such a large, powerful breed it was important to us that she was very well trained, which is why we went for the extra 1 on 1 lessons with someone who was experienced with her breed.
Usually, we just attend group lessons and train at home, but she was our 1st giant breed and we wanted to do things right.