Start at Oxbow Public Market, ideally around 10:30 or 11:00 a.m. The market is compact, colorful, and easy with a stroller, with plenty of natural opportunities for family photos among the produce stands, flower displays, and food counters. Plan about 90 minutes and roughly $15–30 per person for lunch; arrive early on Labor Day weekend for the best parking and easier movement with your infant. The market generally opens in the morning, but confirm holiday hours before leaving.
Walk or make the short drive from Oxbow Public Market to the Napa Valley Wine Train station in downtown Napa—allow about 5–10 minutes, depending on parking and boarding procedures. Book well ahead and confirm that your specific Labor Day departure permits a four-month-old; policies and seating arrangements can vary by excursion. The roughly 2½-hour journey is a good fit for grandparents because it is mostly seated, with elegant interiors and changing scenery for relaxed multigenerational photographs. Bring feeding supplies and arrive at least 30 minutes before departure.
After returning, spend about 45 minutes at Archer Hotel Napa, whose polished lobby, stair details, and downtown setting make attractive indoor or shaded photo backdrops. Rooftop-area access may be limited to hotel guests or diners, so ask staff politely and keep the session unobtrusive. Then drive north to Napa Valley Museum in Yountville—roughly 25 minutes via CA-29, traffic permitting—for a quieter final stop. Allow an hour to view the local art and history exhibits; it is generally manageable with a stroller, but verify Labor Day hours, admission prices, and any holiday schedule before going.