I have a nearly 4 month old who is similar.
The bus is great. He likes to stand on my lap to look out the window or look at other passengers. It also tires him out and he will fall asleep on my lap or in the pram.
Putting him in the upright pushchair instead of the bassinet is partly good. He likes it more because he is able to see more ... But then he wants me to carry him so he can see even better (or to sit on DH's shoulders!), so gets upset and cries.
He now hates the carrier because he can't see and I'm not sure if he's big enough to be forward facing yet.
Stay and play type baby groups are great. He likes to look at the other kids, so the cheap/free church hall ones with a mix of ages are way more successful than expensive, structured ones so far. Was also quite impressed with a local city farm.
At home I've got the high chair out for him to sit in in the kitchen. That's a big success because he's high up and upright which he likes and it has a harness so he can't fall out. He's happy to sit in the morning and watch me narrate 'games' like unloading the dishwasher. He also likes to watch me eat, so making and eating lunch can keep him entertained for quite a bit. Doesn't work when he's tired and fractious though - then he wants to be carried all the time.
He likes to sit with me in front of a big mirror and look at our reflections. He also has a 'bumbo' type seat with an activity table and will play in that for 5-10 minutes with adult interaction. Really enjoys physical play like being swooshed around or 'thrown' up in the air. My capacity for these games is pretty short though because he's heavy!
Other people are a big hit. He had the time of his life when my in laws visited and he had several other people on hand all day to bounce him around and make him laugh. It sounds like visiting his cousin and family is also popular with your DS?
I've also found some sensory toys helpful for a bit of wind down when he is getting tired - he likes touching a tasseled shawl, some very soft Jellycat cuddly toys and a toy with crinkly textures, ribbony bits etc is great for when he's getting sleepy in the pram but still wants to be doing something.
All that said, last week was a total was a total mess of overtiredness and refusing to nap, so sometimes nothing works well!