If it's for a ton of people I would say getting fancy will be a nightmare. Quantity over fanciness will be more appreciated and if you're having a barbie it would look a bit silly to have tiny bites of things that you'd have to spend ages assembling and wouldn't be able to spend time with the guests.
I would go for casual and generous, with things you can easily make beforehand that don't need any assembly. Really I would just have:
Big bowls of tortilla chips, pita bread, chopped vegetables and one or more dip - salsa (if you want you can make your own), mexican layer dip, white bean dip, cheesy spinach dip and or crab dip
Big platter of shelled prawns, US style cocktail sauce (combine 250ml ketchup, 1 tablespoon creamed horseradish, salt, pepper, chili powder, paprika and lemon juice)
Or instead of (or in addition to) the more American style dips I suggested, you could go Mediterranean - bowl of hummus, tzatziki, taramasalata, olives and feta.