So, you have 10 watts available. As a starter I'd suggest you use a 12 volt bulb. A 20 watt halogen uses just under 1 amp when powered from 5 volts. Alternatively a 4.7ohm power resistor will do the job. Just make sure it is rated for a dissipation of at least 10 to 15 watts . Although it'll only be dissipating 5 watts you should always under-rate resistors to prevent failure.