In short
Interlaken is, as the name says, between two lakes: Thun to the west and Brienz to the east, separated by an alluvial fan on which the town sits. It has no remarkable old town: it is a base, not a destination in itself.
What you come for is around it: the Jungfrau region, with Lauterbrunnen and its valley of seventy-two waterfalls, Grindelwald, Wengen and Mürren — the last two car-free.
Arriving and getting around
The train arrives from Bern or Lucerne in under two hours. Two stations: Interlaken Ost is the one for mountain connections, Interlaken West is closer to the centre.
Mountain transport is expensive and is the real budget. The Swiss Travel Pass, the Jungfrau Travel Pass or the guest card depending on your accommodation change the arithmetic considerably: compare before buying individual tickets.
What it really costs
The region is among the most expensive in Switzerland.
Staying at Lauterbrunnen or Wilderswil rather than Interlaken itself often costs less and puts you closer to the trailheads.
When to come
June–September for walking, December–March for skiing.
May and November are best avoided: some lifts close for maintenance and the weather is unsettled.
What nobody tells you
The Jungfraujoch, "Top of Europe", is at 3,454 metres and one of the most expensive train tickets in Europe. The journey takes about two hours from Interlaken. In overcast weather you pay a great deal to see fog: the summit webcams are public and can be checked that same morning before deciding.
Alternatives give comparable panoramas for far less: Schynige Platte, the Harder Kulm just above the town, or the Männlichen.
Interlaken is one of Europe's great centres for adventure sports — paragliding, skydiving, canyoning, rafting. The offering is dense and the operators numerous; check the certifications.
Lauterbrunnen is an inhabited village that has become very busy: the commune has introduced measures to manage visitor flow and parking.