Think about what the boys are interested in. If they live out of town and you don't see them enough to be up to date/confident on that - check with your bro. Choose a related experience that offers sensory friendly days. Make sure that the venue has facilities to enable feeding the newborn and doing nappy changes so that they are able to stay longer.
Depends on how old? And what are his special interests? If he's into trains maybe try a train ride to a train museum if that works where you are. If he's into sharks maybe an aquarium etc. and like Raqi said they often have sensory friendly days too!
Zoo pass?
Aquarium?
Zoos and museums in my area sometimes have sensory friendly days, maybe you can look into gifting them tickets for an event like that?