Bandar Abbas is hot. In summer (June to September) temperatures regularly exceed 40°C, and the humidity from the Gulf makes it genuinely difficult. Avoid July and August unless you have no choice. The best time to visit is November to March, when temperatures are pleasant — around 20–25°C — and the evenings on the corniche are comfortable.
The islands of Qeshm and Hormuz are best visited in winter and early spring. Qeshm in particular is worth two or three days: the Hara mangrove forests, the Valley of Stars geological formations, and the ancient village of Laft are all rewarding. Hormuz is smaller and more atmospheric, with its distinctive red-oxide soil and crumbling Portuguese ruins.