Hello,
We are traveling to Belize for 7nights in March. We are 4 adults and 3 teenage athletic boys, we will rent a van from Belize City airport and for accomodation we will consider an airbnb.
We love nature, adventure, beaches, fishing, snorkeling, good food and to imerse into local culture.
We are debating over staying in one place only, or two places - is it too short spending 3nights in San Ignacio and another 4 nights in Placencia? If it's too short, what (? other) destination do you recommend?
Thank you in advance.

No, I think that splitting a first, quick visit to Belize is a good way to go. You get a bit of jungle and a bit of blue water and the things you want to see are closer so easier and more fun to access. The whole time in Placencia with a van would work but many of the things I'd want to take in (ATM cave, Maya sites, the reef) are relatively far away. I'd consider a split between San Ignacio and either Ambergris Caye or (our favorite) Caye Caulker because they lie close to the reef which is pushing 20 miles off the coast of Placencia. You'd need your rental for less time, too, if you even still want to rent. Have a great trip, whatever you decide!