- An old-fashioned picnic in a park that allows them. Try to borrow a traditional wicker picnic basket if you don't have one, you just need disposable paper & plastic wear, and a small cooler for some iced drinks. A state park might have picnic sites with tables & grills (bring charcoal), hiking trails, scenic views, perhaps a boat or kayak for rent. But like some house cats, an indoors kinda guy might not like this.
- Free or low-cost museums (art, natural history, science, etc). I can get engrossed in one for hours, and as you go around you compare comments on the things you see, a good way to bond with some guys, get to know them.
- Attend a free or low-cost public event, like a street fair, a gay festival around Pride time, a convention center show (auto, home improvement, garden, etc). The OP has mentioned SF Carnaval next weekend, and that's the idea.
Be armed with several ideas in case he rejects some. "Been there, done that." "I hate museums." "Boring." Letting him contribute some suggestions, and deferring to his wishes can also get his enthusiasm up. You don't wanna make this seem like being dragged along on the involuntary family outing from when you were a kid.