Lonavala has two very different personalities depending on the season, and which you prefer depends on what you're after.
Monsoon (June–September) is the most popular season and the most dramatic. The south-west monsoon arrives in early June and transforms the Sahyadri escarpment: the brown basalt hillsides turn luminous green, waterfalls appear overnight on cliffs that were dry a week before, and the air cools to a pleasant 20–25°C. Tiger's Leap and Bhushi Dam are at their most spectacular. The trade-offs are the crowds — Lonavala is extremely busy on monsoon weekends, particularly July and August — and the rain itself, which can make fort treks slippery and roads unpredictable. Weekdays are noticeably quieter than weekends.
Winter (October–February) is the better season for trekking. The rains have cleared, the landscape is still green in October and November, the skies are sharp and clear, and the temperature is cool without being cold (10–20°C). Lohagad and Visapur forts are much more enjoyable to climb on dry paths. The Karla and Bhaja caves look magnificent in winter light. This is the quieter, more comfortable season.
Summer (March–May) is the least appealing time. Temperatures climb to 30–35°C, the landscape turns brown and dry, and most waterfalls disappear. The altitude provides only slight relief from the heat. Unless you have no other option, avoid.
Year-round, weekdays are significantly quieter than weekends — Lonavala's crowd problem is a weekend problem, not a seasonal one.