Prepare ahead of time. Short of using an elastic load balancing service, predict when you may have traffic spikes, and choose a cheap-to-implement solution such as request-level load balancing.

Create replicas of your web server processes on different machines for the duration you expect a potential traffic spike. Use a fast dispatcher like nginx[0] as a reverse proxy to load-balance requests to the appropriate replica web server machine.

If you see consistently low traffic, spin down the replicas and remove them from your load balancer configuration.

[0] http://nginx.org/en/docs/http/load_balancing.html