An electronic unit incorporating an LED or a sound source, a battery power source, and a displaceable actuator to connect the LED or sound source to the power source, is mounted inside a non-latex balloon by firstly being bonded by means of a cover sheet to the exterior of one of the flexible strips of a self-sealing valve located in and through the neck of the balloon. This may take place by heat sealing and simultaneously with the bonding together by heat seal of the respective elongate strips of the valve. Thereafter, the combined unit and self-sealing valve are bonded, again preferably by heat sealing, between respective balloon sheets in the vicinity of the stem of the balloon. Winged portions on the inflation valve strips and the cover and corresponding bulges where the balloon body emerges to the balloon stem may be provided.