When the situation permits a casual ensemble, dress in a light blue polo and beige cargo pants. Why not introduce a pair of espadrilles to this look for an extra dose of refinement? Relaxed casual ensembles for 30-something Millennial gents aren't actually that hard, as you can see here.
If you're on the hunt for a casual street style and at the same time on-trend look, marry a navy crew-neck t-shirt with beige cargo pants. For footwear, you can go down a more elegant route with a pair of espadrilles. Those who wonder how to sport cool off-duty dressing as you pass the big three-oh, this pairing should answer your question.