Are you able to put down bricks or cobblestones? Or Pre-cast concrete pavers?
Ideally you want a hard surface that won't be muddy in the rain, and will allow rainwater to drain off. It should therefore stand slightly higher than the nearest drain.
Bicycles aren't heavy, but you still want a flat base layer of compacted sand to support the pavers/bricks, to prevent them moving and cracking. The base layer would need to resist washing out in rain too. A good base layer helps keep all your pavers/bricks level to prevent trip hazard.
Lastly, you will want some way to lock up your bikes too, which probably means some kind of hasp or loop or shackle to secure your bike's lock. I'd also put hooks in the inside for things like helmets and wet coats.