If the situation allows a casual look, consider wearing a navy short sleeve shirt and white chinos. Boost the formality of your outfit a bit by finishing with dark brown leather loafers. Totally summer-appropriate, you can rock a version of this ensemble all summer long. Casual looks for 30-something men aren't actually that hard, as you can see.
This combo of a navy short sleeve shirt and white chinos is perfect for most casual settings. A pair of white canvas low top sneakers acts as the glue that will bring this getup together. You're guaranteed to always look stylish even despite the scorching heat if you keep this getup as a last-minute grab. If you're not sure how to dress your age but also look fashionable, this combination is a practical illustration.