What's the best way to extend the range of my Airport Express?

The "best" way would be to connect a second AirPort Express to the first by Ethernet and configure both of them for a roaming network. This would provide you with the best overall bandwidth for your wireless network.

Then next best would be to connect a second AirPort Express by a wireless connection. To get the most optimal bandwidth in this configuration, you need to carefully place the second Express. Please check out the following AirPort User Tip for details on base station placement.

The best way is to connect them to one another with an Ethernet cable.

If that is not an option, consider a pair of MoCA adapters, or powerline adapters if your garage shares electric power with the house.

If that is not an option, a "wirelessly extended" network may be acceptable. There is no way to determine how well it will perform until you try it.