You can line the walkway with tealight candles - you can get a pack of 100 for about $5 from Ikea. You can also get cheap holders from various places although you could place them in the sand just a little way down to protet them from any wind but will still give a lovely glow.

For back ground decorations, you can hire free standing candlearbras (sp?) which might look nice. We had wrought iron ones with lovely chunky candles in the back ground at our wedding (got married at a winery though). If you get married at dusk or something you can light them.

You could also hire a cheap marque and decorate with flowing scarves or other material. You can hire cheap wrought iron freestanding arches as well.

Hope that helps.

MG