Two days is enough to get a useful feel for San Bernardino without pretending you have seen the entire Inland Empire. I would divide the weekend between the city’s Route 66 story and the mountains above it.
Day one: Route 66
Start with the Original McDonald’s Site, then follow the Route 66 theme through town. Stop for lunch at a long-established local Mexican restaurant rather than a freeway chain. If opening hours work, add the San Bernardino History & Railroad Museum.
In the evening, check whether the California Theatre has a performance.
Day two: go uphill
Leave after breakfast for Lake Arrowhead or another part of the San Bernardino Mountains. The drive itself is part of the experience as the valley falls away and the landscape changes.
If mountain roads or weather are poor, substitute Glen Helen Regional Park or nearby Redlands.
My main advice
Do not overfill the schedule. Distances that look short can take time in Southern California traffic, and mountain driving is slower than freeway driving. Give the city a proper day, then take advantage of the very different California beginning immediately above it.