This post was inspired by a conversation I was having in a group on Facebook, plus the fact that schools in England and Wales are poised to go back this week. Mind you, Reception and Year 1 get free school dinners now, so the youngest probably won't be taking their own lunches, but still. Adults like to take sandwiches and wraps, too - and one's own are so much nicer than bought, even if it's nice to buy them occasionally. Who has time to make their own BLT of a morning?
So you start with the bread. I tend to always use bought bread for a sandwich, but if you can slice your home-baked loaves thin enough, go for it! My personal preference is a seeded wholegrain loaf. Pitta bread or tortilla wraps are nice for a change, too. Also, now that Lidl do such delicious rolls, baked fresh each day, I'll often go out and buy one specially (Lidl is all of 50 yards away!). But then, I tend to make my sandwiches when I want them; for lunchboxes, I would find a roll difficult to manage.
If whatever you are using for a filling doesn't spread readily, you might want to use a little butter (or equivalent, if you're vegan), but if it's something like cream cheese, it doesn't need it.
I divide sandwiches into two - the main event, as it were, and the garnish. The garnish is something vegetable - tomatoes, cucumber, sliced peppers, avocado, lettuce, grapes, any or all of the above! Even banana can be nice, especially with peanut butter (although that is a combination I prefer in a breakfast sandwich). If you're making your sandwich to take to work, avoid sliced tomatoes and cucumber, as they can make it soggy; use cherry tomatoes instead, and take a hunk of cucumber to eat separately. Oh, and don't forget pickle (or chutney) with a strong cheese.
The main event can be all sorts of things - hummus or peanut butter if you want a vegan sandwich, or all sorts of different kinds of cheese, including cream cheese (with or without Marmite) and cottage cheese. Or egg mayonnaise - I always put chopped spring onions in mine; my mother uses chives to the same effect. Grated cheese and carrot, bound with a scrap of mayonnaise, works well, too.
I was thinking in the supermarket that you could sprinkle sunflower seeds into your sandwich for extra crunch - Lidl sells them at the checkouts, which I find far, far more tempting than the sweets they've replaced! Ah well.
For omnivores, of course, there is pate, there is ham, there are all sorts of proprietary sliced meats, or you could use some cold chicken (for instance) if you have some. Bacon is good, but nicest when eaten freshly cooked, so we save our bacon and avocado sandwiches for the weekend. Cold sausages work well, too. And don't forget smoked salmon, which can be bought very cheaply nowadays - with cream cheese and avocado, it is a feast! Or, if you like tinned fish, you could always mash some up; not sure how well they would travel, though.
If you get sick of sandwiches, as we all do sometimes, there's plenty of other things to take. Salad is always good - what works best is to put the "nice bits" (chopped tomatoes, cucumbers, peppers, scallions, avocados, sunflower seeds, etc) with the dressing into one container and keep the greenery - lettuce, Chinese leaves, baby spinach, rocket, etc - separate, combining them all at the last minute. You can buy - or make, if you're that way inclined - all sorts of nice bits for protein: falafel, pork pies, quiches, even a Cornish pasty (nicest hotted up, so I hope work has a microwave - if it does, you can take a mug of soup, too; they sell special mugs to take soup in these days. And if you have a shaming taste, as I do, for ramen noodles.... sometimes I cook those in the microwave and then poach an egg in them, which is lovely! Not very good for you, mind, but still lovely!
Then there are all sorts of rice salads or couscous salads you can make or buy to eat. Home-made is often nicer, but I do rather like bought couscous salad! And sometimes I like a box of (preferably veggie) sushi as part of my lunch!
All very vague and off the top of my head.