Are there tent alternatives?

I don't think you have any alternatives unless there are trees you can move the ceremony to.

I would find out what the exact reason for not having a tent is. Because if you just cant have anything in that space then there's not really much choice. But if you just cant put anything in the ground then you might have more options.

View Quoted Comment If they let me put up something just not in the ground, what are my options? I thought of maybe giving out umbrellas and fans? But I wasn’t sure. I thought of a canopy but don’t those go in the ground too?

View Quoted Comment If they let you put stuff up, just not on the ground, you could attempt to secure something like "sail shades" to the trees. That's the only thing I can think of other than providing bottled water and parasols and/or fans as favors.

You could get those canopy tents and secure it with sandbags or something rather than putting it in the ground. The only reason they have you put that stuff in the ground is to make sure it doesn't blow away. Fans or umbrellas are cute too though and I love the sail shades idea.