Friday Night BBQ Party
Use a large boned shoulder of lamb for this. You can use boned leg if you wish, but it's not as good. This recipe is a backwards one, as you marinade the meat after cooking it. It's handy barbequing a shoulder like this, as it is an uneven shape, and will give you some very pink slices as well as browner meat for those who like it more well done.
Keeping the lamb in one thick flat(ish) piece, (not rolled and tied up like a roast), cut as much of the fat off as you can, otherwise it will create lots of flames as it drips on the coals. Barbeque it for around 20 mins, maybe a bit longer, until it is pink inside.
Meanwhile chop up a few generous handfulls of flat leaf parsley and mix with several generous glugs of olive oil, lemon juice (about half of a lemon), paprika (about 1 or 2 teaspoons) and cumin (about 1 teaspoon). Chop one or two cloves of garlic finely and fry gently in olive oil until it goes a pale golden colour - mix this in with the rest of the marinade and add salt and pepper. All these quantities are only rough estimates of what I put in, so do it according to your own taste.
When the lamb is cooked, put it in a heat proof dish, cover with the marinade and leave for twenty minutes. I recommend that you put the dish on the edge of the barbeque, so that the lamb doesn't go cold. If you do this, it's worth slightly undercooking the lamb, so that it doesn't over cook whilst keeping warm.
Serve with salad and new potatoes or couscous.
I'm going to assume that most replies here will address the meat, and that you'll have that taken care of. To have WITH the BBQ, that's the real question. Depending on which direction you choose, you could go with the Asian, Carribean, or American routes. If you go American, one of my favorite sides is a garlic mayo potato salad, with horseradish and green onions. (with corn bread, baked beans, caramelized onions, strawberry shortcake or apple pie - that kind of thing).
Start with red potatoes, skin on. Smash up a few cloves of garlic, add some kosher salt and olive oil, mash with knife to make a puree. Throw in a bowl, add dijon mustard (1 Tbsp or so) and mayo (couple spoons), grated horseradish (from a jar is easiest), salt and pepper to taste. I eyeball everything, but in general, I go heavier on the olive oil and mustard than most mayo-based potato salads. Your mixture should be creamy and it should pack a good punch. Add your potatoes to the bowl while they're still a little warm, and mix to coat. They'll soak up some of the wet ingredients. Add a handful of chopped scallions, both green and white parts, and some fresh herbs. Basil and parsley give some good color and they keep it fresh. Crumbled bacon is a nice optional addition, depending on the level of carnivory of your guests. Well, it is a barbecue...
You mentioned fish. A cedar planked salmon fillet is always a tasty option. Smokey, moist, easy too. Depends on how much variety you want to go for.
For an Asian approach, (more soy based marinades for your flank steak) a spicy slaw with red and white cabbage, ginger/sesame/soy dressing could be nice. If you go Carribean, you can't go wrong with mangoes, or grilled peaches (with fresh yogurt and mint, to borrow a Bobby Flay approach). I also like a platter of grilled vegetables on the side - zucchini, red peppers, and eggplant are favorites.
Enjoy, and have a great party.