I have a 17 week old springer who doesn't go into the garden alone either. I sort of assess how much damage the thing in his mouth can do to him, iyswim?
Stones, plastic, plants etc get extracted, the odd leaf, twig, grass etc I leave him with, which probably isn't the correct thing to do but, it's like when your DC are little and you give them all manner of toys but they want your keys or whatever!
He can spend a long time arguing with the watering can but generally just sits and watches me! If I throw a toy or ball he either isn't interested or will get it and run off, ending the game. I need to teach him to bring it back as i would happily play with him for ages!!
It's hard to find stuff to do at this time of year, today I swept up, did some weeding, filled the bird feeders up again. Otherwise I stand there and he sits there and watches me! I wave all manner of toys and things around and jump around like a loon, as soon as I stop, he just goes back to watching me, sat near me. DH says just let him and get on with your stuff, if he's not interested in playing with you or his toys!!