Soft play is fine and at these ages they like doing the same thing all the time 🤷♀️
Mostly at that age we did stuff like staying at home and building wooden train tracks or Duplo, doing big chunky jigsaws, building an obstacle course in the front room, setting up a pretend restaurant with toy food, play doctors with a doll, teddy etc.
9mos just like to touch stuff and put it in and out of other things, experiment with it, push it around/over, whatever - whereas 3.5 is a great role play or small world play age. You can usually combine these if the 3yo isn't too frustrated with the baby getting into their stuff but not understanding the "rules".
Yes there is a lot of 10-20 min activities - I would alternate these with stuff like letting them free play, some TV, doing some washing which they can "help" with etc. We try to get some fresh air every day even if it is wet, but some days I don't bother. Some good stuff to do when you're bored of the park - find a local river/stream with ducks to feed (not bread - look up what they like, I forget), local farm parks, go for a walk near a field with sheep/horses/etc (don't feed these!) Or there are different kinds of parks which are better for wet days - maybe a maze? Walk to a local shop also gets the fresh air box ticked and something useful done, even though it's boring for the 3yo - you can bribe them by offering to buy a treat, give them a job to do, do a hunt for letters/colours.numbers etc.
Swimming can be great though I only do this when I have a 1:1 ratio - I don't think it's safe to take 2 DC.
Tripadvisor is a good way to find local attractions.
I also liked meeting up with other families. Alternate houses if it's too wet to play outside, and the DC get to see new toys plus the benefit of a playmate their age. You get an adult conversation and you only have to tidy up half the time. I have found this harder with DC3 (2.5 so maybe similar to your eldest) as he was born during corona and I haven't really met other parents with DC his age.
If you're at home with the DC during the week, I went to a lot of toddler groups. That fills a good chunk of the day and we would do an errand on the way home e.g. popping into local supermarket.