The Tiber is the one straight line through the middle of Rome, and the stretch from the angel bridge down to Ponte Sisto is the best way to walk it: monuments on the bank, the river below the traffic, and a pedestrian bridge into Trastevere to finish. Do it towards evening, when the light comes off the water and the travertine warms up.
Start on Ponte Sant'Angelo, lined with Bernini's angels, walking towards Castel Sant'Angelo — Hadrian's tomb turned fortress turned prison, the most reused building in a city that reuses everything. Go up for the ramparts or read it from the bank; both work. Then drop to the lungotevere (the embankment) below the high flood walls, where the traffic vanishes and it's water, plane trees, and the odd rower.
Walk south with the current. Around Ponte Vittorio Emanuele II, look back upstream — the dome of St Peter's lines up over the bridges, the view people queue for, here for free. Follow the bend past Lungotevere dei Tebaldi to Ponte Sisto, the fifteenth-century footbridge into Trastevere. Cross at dusk, stop on the middle, look back. The tram out is on the far side.
