Your toddlers aren’t the problem, OP. You are. You turning your nose up at the shabby or free things everyone else uses.
My son was a fractious ball of energy at 3.5 and I had younger ones in tow, so I understand the need to keep him busy. This was the cheapest age to entertain in my experience, because the most boring thing is exciting when you’re little.
A day rider bus ticket and a packed lunch once a fortnight is A Grand Day Out for a 3.5 year old. Ride the top of the double decker playing colour bingo with cars or Nice/Not Nice by waving to people, a stop at the library to pick books and read a bit, back on the bus to a park with swings, picnic lunch, bus to the market for food (the fish mongers was an endless source of amusement - especially when there were live crabs or lobsters). Walk near the train station to either chase and annoy feed the pigeons or go to Pets At Home to say hi to the guinea pigs. Sometimes get a platform ticket to gamble on train lines (each pick a platform, each time a train come to the platform you get a 5p on your pile, winner chooses the sweet) and usually talk to a train driver or two who were very friendly to train-obsessed preschoolers.
That was our alternate Wednesday for years. I loved Wednesdays because he was knackered and went to sleep early, so it was my movie night.
Your son doesn’t need entertaining, he needs something to do. This may not be interesting to you, but it certainly can be cheap.