c:geo is a simple yet powerful unofficial geocaching client for Android devices. In contrast to other similar applications, c:geo doesn't require a web browser nor file exports. You can just go geocaching with your phone and without any home preparation or worries.
CalendarView is a highly customizable date picker, that allows multi-selection.
Retrofit is an wonderful network library to deal with api's and Rx java can be used to do async task operations effectively. Using retrofit with RxJava will results in seamless network calls in mainthread. Retrofit can be used without RxJava, though to perform huge api calling and to do api calls in background, they pair a better combination.