[rmc]

You don't need an EBS to run an instance on EC2.

You can just run the instance as normal. It has a root filesystem, you can write to it etc. However if you stop the instance, then all your changes are lost. The EBS is only needed if you actually need disk space.

[arst]

Micros (which are what you get on the free tier) are only available as EBS-backed instances.

[deno]

It's worth noting that 20GB EBS space is provided with free tier. Though all default Ubuntu AMIs use 25GB EBS by default AFAIK.