October to March is by far the best time to visit, with warm days, cool nights and comfortable conditions for exploring the fort, souq and mountains. Spring (around March and April) adds the damask rose bloom on Jebel Akhdar. From May to September the interior is extremely hot, often above 40C, making midday sightseeing hard, so if you come in summer, do everything early and rest through the afternoon.
Nizwa sits inland and gets very hot from late spring through summer, regularly above 40C with strong sun and little shade at open sites like the fort and falaj. Carry plenty of water, wear a hat and sunscreen, sightsee early or late, and rest during the hottest afternoon hours.
The road up to Jebel Akhdar is steep with sharp bends and is legally restricted to four-wheel-drive vehicles for safety. Do not attempt it in a normal car. Use a low gear on descents, watch for oncoming traffic on tight corners, and take a warm layer as the mountain is much cooler.
Nizwa is more traditional and conservative than Muscat. Modest clothing covering shoulders and knees is expected for everyone, women may prefer to carry a scarf, and revealing dress draws unwanted attention. Respect prayer times, ask before photographing people, especially at the goat market, and behave discreetly around mosques.