Is there any way you can drill holes in the bottom of the benches? You will have trouble growing properly without good drainage.

I would probably line the thing with heavy plastic or a cheap pond liner on the sides and bottom. Damp soil sitting next to the wood would hasten rotting.

I think it could be really pretty. California is so different from here, I'll let someone else make plant recommendations for you. Have fun!

SW,
Any chance you could post a photo? I'd echo what Watergal said and therefore, maybe containers on top of? or inside? the benches might be a better way to go?

Seacat

If you really don't use the benches, why don't you just get rid of them? Then you can put whatever containers you like there without having to worry about rotting the wood or having enough drainage in the bottom (both of which I see as potential problems). You can also move the potted plants around to suit yourself instead of always having them in one spot, which you can't do if you use the benches. Anyway, just a thought.
MacThayer

i think your idea sounds great and could turn out beautiful and handy too.you do need some drainage holes though.