Although the adjective full ends with a double ‘l’, adjectives like useful have only one ‘l’:
✗ Find a more usefull spare time activity than driving around in a car.
✓ Find a more useful spare time activity than driving around in a car.
Other adjectives ending in -ful are also written with only one ‘l’, for example:
careful, faithful, harmful, helpful, meaningful, painful, powerful, skilful, successful
Note however that adverbs formed from these adjectives are written with double ‘l’, for example:
carefully, faithfully, helpfully, usefully etc