If you want a really easy canapé, pizza pin wheels are really easy and tasty - roll out ready made puff pastry into a rectangle, then spread with tomato. (a tin of chopped tomatoes boiled down to a thick smushed with lots of garlic and a pinch of sugar is nicest tbut tomato purée, sun dried tomato purée, or even ketchup can all work if you're in a hurry). Then roll it up like a Swiss roll. With a sharp knife, cut into slices about a cm thick and lay on a baking tray, then sprinkle with grated Parmesan and some oregano and cook according to the time your puff pastry needs as per its packet. Yummy served hot or cold.
You can also fill with other things - eg pesto and then sprinkle with Parmesan and pine nuts, or ricotta mixed with garlic or horse radish and sprinkled with paralysis after cooking etc. And roll them up into a long sausage of different shapes - like palmiers or heart.
Little sausages as said go down well. I did a sit down meal for 20 at Christmas and did them as an extra to lasagne. They were late coming to the table (oven playing up) and even then, when people were complaining that they were full, they still managed to demolish about 80 of them. People always seem to eat lots more of them than you think. I've tried loads of different mini sausages over the years but keep coming back to the Waitrose essential ones. They taste great, don't exude loads of water or grease during cooking and at Christmas cost £4 for 2 packs of 30 so they're comparable in price to the other main supermarkets.
A savoury cheesecake can be nice and a bit different on a buffet - use a mixture of plain digestives and a few nice crackers with a bit of finely grated Parmesan and/or choppedtoasted nuts and seeds for the base, make up as you would for a normal cheesecake. Then things like Roquefort, Parma ham and pear; feta, cucumber and mint; Stilton and onion marmalade; google savoury cheesecake and loads of different ideas will come up.
A layered galette also looks nice - use pancakes or tortillas as the layers and then fill with whatever you fancy as if a lasagne. Sprinkle with cheese and bake - use a springform cake tin for ease of getting it out - and then you have another hot or cold savoury dish that can be cut like a cake that looks and tastes good.
Have a look at a Spanish tapas book - they are the masters of serving up hot food For east eating. Thus instead of lots of sloppy mince/chilli/ragu etc they do meatballs in a piquant tomato sauce. And patas bravas a potatoes - bite sized sautéed cubes of potato with separate spicy tomato sauce and aioli to spoon on as you wish. So no awkward cutting if eating from your lap and you can make the potatoes as spicy or garlicy as you want. Likewise lots of stuff is served on sticks as mini pinchos (sp?) so easy to eat away from the table.
It's also worth picking up the entertaining food ordering booklets from waitrose and m&s - just for ideas (may well be online too) for nice ideas or ways to tweak or present ordinary stuff to make it special for the buffet. So things like adding capers and fominely chopped mini gerkins to a potato salad. Or having some pomegranate seeds to hand to go through a salad to glam it up.
If for pudding you do dipping fruit or ice cream or anything that needs hot chocolate sauce, this is the worlds easiest and tastiest recipe - melt 1 large bar of bournville or other nice dark chocolate (approx 200g), broken into pieces, along with a walnut sized knob of butter, a pinch of salt and about a tablespoon of honey together. Wait for it to cool down a little then gradually stir in a small pot of double cream. And thats it. Warm it up again when needed - it keeps for a week or more in the fridge (assuming you don't take a fingerful each time you are in the fridge to check it is still ok . You can also tart it up by adding stuff to it - make it a Jaffa sauce by adding some marmalade (without rinds) and orange zest. Or a little bit of a nice liqueur or spirit to complement what it is being served with - add slowly to taste as you can always add more but can't take any excess out.