You could do a treasure hunt in the garden, with individual pieces of lego for the guests to find (just have a stash to top up in case some if the guests are hopeless finders LOL).

Also, you can hire Giant lego from some party places.

And I reckon square or rectangular cakes with 4 or 8 mini smarties would look very lego-ish. Or you could do rectangular mini-pizzas with 4 or 8 round bits of kebana or sausage in the right place too.