Oh what about picking berries!?
Definitely something I have on my Summer plan for days out. I wasn't even thinking of it being fine motor, just a nice experience and a trip for us all.
I'm really looking for things at home indoors when we have rain, the last couple of days it's been like tropical storms after the heat wave. My 'big plan' for summer is huge amounts of out door time and as much gross motor as possible so even on rainy days I'm trying to take him to indoor soft play and swimming, but the cost is too much to do all the time (plus a bit repetitive even with doing a rota of play parks and pools) and I have a hope for just one day per week where we don't go out, yk just chill together at home. I doubt we can manage it tbh.
We made a helicopter this afternoon which went down brilliantly. It's a construction set he was given (free cycle) with nuts and bolts, spanner, screw driver etc and I really didn't think he was up for it, but suddenly he is

I think the motivation came from me buying him a toolbox (again £ shop - wow I sound like a cheapskate!) this morning and he got all excited to put his tools in it, then asked to build the helicopter he saw in the instruction picture. A full 30 mins focus with me helping. Then we did a 'work book' page together, a shapes and opposites book I picked up in the £ shop this morning which he really enjoyed.
He also had another go with the wooden pegs so that was great too

I would totally love to do gardening with him but we don't have a garden, our roof terrace is so hot on hot days and gets full on gales during the storms which just batters plants to dust. I have set up an indestructible out door toy (play box with stones instead of sand, bucket, rake, wheelbarrow, watering can - yes he puts stones in it and pours them out - treasure stones to dig for, dinosaurs etc etc) but that's about our limit for outdoors at home.
I think sewing would be too fiddly for him yet. He still gets frustrated by the the big chunky beads and fruits at play group with fat string to thread through. I give him cotton reels and straws for threading at home and he's agreed to use the string once for those, I really don't think he could cope with smaller beads or a needle yet even a fat needle.
I think part of my search is something that is just the right level for him because he is SO INTENSE and gets very frustrated when he can't do something, I don't want to keep offering things that upset him even though I am happy for him to feel the drive of frustration and desire to want to master his skills.