Mid-weight merino wool or synthetic baselayer (1 if you run hot, 2 if you run cold). Baselayers are the tops closest to your skin.
Midlayer, such as a sweater or puffy jacket. Down puffy if you run hot, synthetic puffy if you run cold (or if you're a vegan.
Outer layer, a shell like a rain jacket if you run cold and/or insulated, water proof/resistant jacket. It does rain in the spring.
No big, heavy coats, because they're not easy to pack in your bag and too heavy to lug around.
Although you can still wear shorts, it's tick season and long pants are recommended.
Cotton or cord pants are OK. Denim not recommended.
Sturdy sneakers or hiking boots.
Flashlight (because it gets dark earlier, this is recommended as standard).
Bug repellent (like DEET).