The town centre is walkable in twenty minutes end to end. The major sights — the Centro Civico, the Cathedral, the lakefront — are within a few blocks of each other.
Buses run from the centre out along Bustillo to Llao Llao every fifteen minutes (line 20) and to Cerro Catedral every half hour (line 55). Pay with the SUBE card, which you can load at any kiosk on Mitre. The buses double as the cheap way to do the Circuito Chico — get off where the views look good.
Rental cars are useful for the further excursions (Tronador, the Seven Lakes road north) but unnecessary for the immediate sights. Local taxis are plentiful and not expensive. In winter, all four-wheel-drive rentals are spoken for by July — book ahead.