Thanks for all your suggestions - was great inspiration. I ended up making an 'assemble your own' Nicoise salad, using grilled salmon, and potatoes tossed with verjuice, olive oil, parsley and capers. Then rhubarb soup by fiveandspice. Was delicious! The kids enjoyed quesadillas and frozen yogurt and mango pops that they helped prepare. Everyone enjoyed the lunch, especially my father-in-law - who loves salmon and rhubarb.

I think the big question is 'what does your father-in-law like? After all, it's his birthday. Maybe some nice steaks or his favorite fish on the grill.

In the summer here, we often do a cold seafood plate - cooked lobster, shrimp, crabmeat, etc. - with several sauces, a nice green salad, good bread and ice cream for dessert...easy and refreshing in the hot weather...

How about a grilled steak salad? And ice cream for dessert, with all the toppings. :)

A super-easy crowd-pleaser is Frogmore Stew, which does not have frogs and isn't a stew but more of a shrimp boil. It makes a spectacular table presentation and it's just fun food. Here's a recipe I've used/adapted many times to great acclaim: http://www.williams-sonoma.com/recipe/frogmore-stew.html?cm_src=RECIPESEARCH. One catch is that you need Old Bay seasoning. If it's not available in Australia, here's a recipe: http://busycooks.about.com/od/homemademixes/r/oldbaymix.htm.

Can you get your hands on some cedar or hickory (or other) planks? Plank grilled fish is also a great party dish ... if you want to mix it up you could do a combo of seafood and maybe some beef or pork kabobs. Pierino's paella suggestion is also a good one - there is a tailgate paella on this site specifically designed for the grill ...

One thing that can and should be cooked on the grill is paella http://www.food52.com/recipes/7244_the_sun_also_rises_paella You should have no problem finding suitable bivalves in Australia. This is my recipe but there must be about a dozen posted to food52.

I'd say definitely some grilled seafood. How about tuna? Just sear it very, very rare on a hot-hot grill and pick sides according to your family's tastes. I'd probably opt for a Salad Nicoise, myself....

This Salmon with Red Curry sauce on a bed of basmati rice with a shower of Asian slaw on top is one of my favorite elegant company dishes. It is pretty easy to make and plates up restaurant quality appearance. I have access to a milder prepared red Thai curry paste that I use when I don't know if people can take a little heat, but this sauce overall is fairly mild.

I like to put toasted sesame seeds in the cabbage salad.
http://www.terrarestaurant.com/salmon.html
pic here
http://whatdidyoueat.typepad.com/what_did_you_eat/2007/05/whb_salmon_with.html