Having given this some thought recently as I'm toying with an "open house christmas celebration" at some point over the festive season, I have decided that if I DO do it, I'm going to stick a massive lamb shoulder (or two) in the oven and slow roast them. I am then going to use my kitchen counters as a big buffet, based on lamb and wraps/flatbreads for people to make themselves easy-to-eat-standing-up, with added benefit that it really doesn't matter if the lamb sits out for a while as people come and go. To add to the lamb and bread I will be making some tzatiki and some baba ganoush and some hummus. I will also be making some pickled red onions (in lime - I can't express how well they go with lamb, aubergine and flatbread! 
) as well as a tomato salad and a tabbouleh salad.
I don't actually have any vegetarians on my list but I'd think the dips etc and salad would work for them too. Normally I'd do some halloumi or something but I don't want to be cooking all the time so that feels like it wouldn't work. Perhaps some big chargrilled slices of aubergine/mushroom/courgette - although the non vegetarians will inevitably snaffle them so you'd have to be generous or explicit that it's for veggies!
I will also be putting out loads of mince pies, christmas cookies, crips, nuts etc.
To add to the festive air, I've been thinking about christmassy folliage, decorations, candles etc.
Of course, I probably won't do it because I live in fear everyone will arrive at once or will stay too long and my not- very-big-house will be totally overrun! I am from South Africa originally and honestly, I really miss summer christmas - it's SOOOO much easier to entertain!