-Nephew will like the Royal Thai Airforce museum (free). Get there by BTS. Or taxi as there are three or more of you.
-Yes to the river taxi ride from Taksin Bridge to Nontaburi (almost)
-Cycle around Kho Kred (from Pakred) or Phrapadeang (the port ferry). Both have bike rentals.
Depends on where you start from? Bang Na? Go south on BTS get ferry (PakNam BTS) across river and get a taxi to Navy museum? (Phra Chulachomklao Fort). Pleasant restaurant.