Show/tell people where facilities are (where to leave coats/bags, bathroom etc) as they arrive.
Have things like crisps, dips, carrot sticks etc out on a table in the garden to pick at on arrival. A beer or other drink pressed into hand on arrival also helps.
LOTS of napkins. You could wrap cutlery in napkins and put in a basket on the table with salads etc so everyone just grabs a little bundle, but also have spare napkins somewhere they can be grabbed but won't fly away - or maybe a roll of kitchen towel beside BBQ or on salads table to pull off a sheet/2 if needed.
Seating options - whether that's garden chairs, rug on grass, cushions on a wall, indoor chairs brought out....and if possible, at least some places to place a plate/glass when both hands are needed for something.
Plenty of salads. And yes, cook plenty of meat but based on how many are ready for it at a time - often doing sausages etc for smallies (and a bowl to hand around to adults ...) first can keep them happy and then you can do burgers/steak/chops for adults in peace. You might need to do a second round of sausages/burgers for smallies later - or persuade them to have lots of carrot sticks, pepper sticks, cherry tomatoes, slices of cheese etc from the salad table.
Pile of Meringue nests, big bowl of mixed berries, bowl of cream (and maybe some chocolate flakes crumbled in another bowl) is an easy way for people to make their own dessert as they choose. Or a cold tart to slice (chocolate, lemon, cheesecake etc).
Try and prepare ahead of time, and run the dishwasher/do washing up before guests are due. So that there is space to put dishes when needed (I usually run it to finish just before guests are due, however half empty it is, and once I have emptied the clean things, restock the tablet so anyone can turn it on once it's refilled later). Use decent paper plates to reduce clearup later. Put a bin where people can find it (like dishwasher, empty that and put in an fresh liner shortly before guests due regardless of how full it is at the end of prep time).
And also, have your serving dishes and spoons and other utensils organised - both clean and easily accessible. (I often make salads the evening before and morning of a party, put in regular plastic tubs in fridge to save space, and decant into bowls at food time - but have the bowls stacked and ready to decant quickly and with a couple of spoons/spatulas there to do the decanting).
And what condiments will you want? Mustard, ketchup, mayonnaise, salad dressing(s), salt and pepper, BBQ sauce, burger relish....decide beforehand, move them together in the fridge (and put things you won't want to the back out of the way), so you can easily grab or tell someone else to grab at the right time. Or put them all together in a spot on the counter.
Lots of ice - if you don't have plenty yourself, buy a couple of bags from the supermarket. And unless you have a fabulous double fridge and tonnes of room, put drinks in a container of ice to chill. If you don't have a proper icebucket for the job, a garden bucket (that has no holes) or baby bath not in use are good - just give them a very good wash so they are clearly clean. Plenty of soft drinks too, and water (cold fizzy water to make up squash is great for lots of DCs, as well as people wanting it straight).