How do I plan a meal without a stove or oven?
Missing the stove and oven is tough. But it sounds like you might have almost everything else you could possibly need.

With the microwave, you can essentially steam and even boil. With the slow cooker, assuming you have it, you can simmer. With the barbecue (grill, for us Americans), you can essentially do one better than roasting - anything that normally bakes in the oven on high heat is probably doable. The panini grill can do smaller-scale things in the same vein, and in some cases even substitute for sauteeing.

The barbecue might be the real hero, though, especially if it's a good-sized one. You can grill/roast any kind of meat or vegetable, and you can even cook a pizza on it. If it's the kind with a top rack, you can cook things more gently too. As derobert rightly suggests in the comments, you can even toss a cast iron pan on it. Failing that, you can also do a lot with foil - wrap things up to cook a bit more gently and keep them from drying out, or put down a few layers, grease it, and throw food on top to get something closer to sauteeing.

So start planning the meal however you normally would - flipping through recipes, starting from an ingredient, whatever floats your boat - and restrict yourself to things that can be done with those cooking methods. Quite a lot of dishes will be just fine. For example, the microwave will cover a lot of simple vegetable dishes. The slow cooker can do a lot of soups and stews. The barbecue can do tons and tons of things. And of course, you can do absolutely anything that doesn't need heat - salads and other raw foods. Don't underestimate that; there are plenty of great cold dishes out there.

And of course, keep in mind that there's really nothing wrong with reheating for many dishes. I don't know how big the microwave is, but assuming it's a decent size, you can prep something at home - think casseroles, soups, vegetable dishes without very sensitive textures, or just think about what works well for you at home eating as leftovers. Or to give a more specific idea, freshly grilled/roasted meat or vegetables plus a nice sauce you made ahead of time at home can make a pretty impressive dish without much actual work at your office.