CubeSats are currently being involved in more and more ambitious missions. One example is B3LSat (Belgium Leuven Liège Louvain Satellite), a double-unit CubeSat developed in the framework of the QB50 network. B3LSat will be provided with two scientific and two technological payloads. Scientific measurements will be performed using an ultra-low power ionization sensor, which will probe the atmospheric gas composition by fingerprinting the ionization characteristics of distinct gases, and an optical micro-sensor array aimed at monitoring the etching rate of selected polymer materials exposed to space plasmas. The first technological payload will be a low-cost attitude determination and control system, featuring innovative tracking algorithms for star trackers and extremely high-capacity reaction wheels. In addition, B3LSat will demonstrate in-orbit a novel control algorithm for propellantless rendez-vous and formation flying based on the differential aerodynamic drag concept. The objective of this paper is therefore to describe these payloads in detail and to discuss the resulting mission and design challenges.