Late spring to early autumn (May to September) is the prime season, with green valleys, open mountain railways, lake boats running and long days for hiking; July and August are busiest and dearest. June and September are a sweet spot for fewer crowds. Winter is cold and quieter in town but a gateway to nearby ski areas, while shoulder months can be wet with cloud hiding the peaks.
Mountain weather changes fast and it is genuinely cold at altitude even in summer, so carry warm and waterproof layers for high trips like Jungfraujoch. Check forecasts before big excursions, as low cloud can wipe out the views you paid a lot to see.
Paragliding, canyoning, rafting and skydiving are the local draw but carry real risk, so choose only certified, well-reviewed operators, listen to the safety briefing, and accept that guides will cancel or reschedule in poor conditions. Do not push to go if you feel unsure.
Lakes Thun and Brienz and the mountain rivers are fed by glaciers and stay dangerously cold even in high summer, with cold-water shock and currents a real hazard. Swim only at supervised lidos or gently from shallow shores, and take care near fast alpine water.