You can safely use oak Quercus robur, sessile oak Q. petraea, Turkey oak Q. cerris, red oak Q. rubra, European beech Fagus sylvatica, hawthorn Crataegus monogyna, Japanese maple Acer palmatum and Indian Almond leaves. The cones from European alder Alnus glutinosa can be used also. All these have beneficial tannins.
Just make sure that they are well dried out and brown. Submerge and soak them by placing a rock over them in clean water for a few days prior to adding them to your aquarium. This will help them sink easier. Also throughly inspect each leaf for any insects or mold. Leaves with mold should be discarded.