Mental activities for Lab puppy

[RoseyHope]

Hello all, I'm wonderfully impressed with the knowledge here on this board so hoping you can give me some ideas.

I'm an au pair for a family who have an 8mth old working line Labrador. Part of my duties is to walk him for 1.5hrs 3x a week, although lately I've been doing the other 2 days as well as MumBoss' social life has picked up. We live rurally so lots of woods, lakes and heathland for him to explore and sniff about.

However I think he's bored/frustrated. He gets destructive (chewing/scratching walls etc) over the weekend if the family hasn't walked him but the bigger problem is barking at the phone, doorbell and sometimes when he's left alone. I've worked on getting his barking down when he's by himself and it's improved quite a bit but still hasn't gone away completely, esp in mornings when I'm upstairs getting kids up and he's in kitchen.

So I would like some advice or ideas on activities I can do with him during the day or on our walks. I do little training sessions with him each day going over commands/tricks, I've been working on his swimming and he will now go into the lake after a ball, but won't go in of his own accord if that makes sense.

I know it's not ideal, but I need things I can sneakily do with him and that the family members won't need to do as well IYSWIM.

Many thanks, sorry it's so long! x