The best hobby project to select is Solar Cell hobby project. As solar cell produce free electricity its good to design a solar electronic project.
Solar cells cost is little bit high. So its recomended to buy small watts solar panels for hobby projects (10 W or 20 W). How to make solar cell ? You can buy solar scraps at cheap price and can tab it if the cells are untabbed. So such cells can be called homemade solar cell.
Solar energy can be used to light a lamp (bulb), run motor, fan, radio, LED etc. So you can buy an appropriate solar cell kit to start with. Solar panels or kit can be bought from solar dealers. There are different types of cells of solar cell manufacturers.
After you buy a solar panel you can find two wires in that solar panel, one red and one black. The red wire is positive and black wire is negative. You can check the voltage and current of that solar panel with the help of a multimeter.
The first solar cell hobby project is to make a bulb glow, put the solar panel in sunlight and connect the positive and negative leads comming from solar panel to the bulb. You can see that the bulb is glowing. Take 1.5 Volt dc bulb to do this project. Try to draw circuit diagram first.
The second solar cell hobby project is making a solar radio. To do this take a radio which operates on battery. Remove that battery and connect the postive and negative of solar panel to positive and negative of the radio. Now you will notice that the radio is working. Select 3V dc radio to do this hobby project.
Similarly you can make solar cell fan, solar cell FM radio, solar cell motor etc.
Thank you.