TCAT (Tompkins Consolidated Area Transit) covers most of the city and runs frequent service between the Commons and the Cornell campus. Regional connections run to Trumansburg, Dryden, Cortland, and Binghamton. Most routes converge near the Downtown Commons — check schedules if you're out late, as service thins in the evenings.
Ithaca is small enough that walking covers most of downtown, but the hills are real. Climbing from the Commons up to Cornell on foot is a 20-minute uphill grind; the TCAT route 10 does it in seven minutes for a couple of dollars. Bikes work well on the flat lakeside paths but are heavy going on the hill streets.