May to October is the comfortable stretch. Daytime temperatures sit in the 20s Celsius (70s and 80s Fahrenheit), the Cuyahoga Valley trails are open, Lock 3 has its summer concert and food-festival schedule running, and the patios are out. June and September are arguably the best months — the heat is reliable but not yet oppressive, and the humidity has not quite arrived.
Autumn (mid-September to late October) is the photographer's season. The Cuyahoga Valley turns spectacularly: sugar maples, beeches, oaks, and hickories on every hillside, with the river canyon as the backdrop. The first weekend of October is generally peak colour.
Winter is grey, snowy, and cold. Lake-effect snow off Lake Erie pushes south to Akron occasionally; January averages -3°C (about 26°F). The upside: Brandywine Falls partly freezes into ice walls, downhill skiing is twenty minutes north at Boston Mills, and the city's holiday lights at Stan Hywet and Lock 3 are genuinely good.
Spring is wet and slow to warm. May, when the dogwoods bloom, is when the gardens reopen and the place comes alive again. Avoid April if you can — it tends to be the rainiest month.