It can be worth your while to prepare something the day before, especially if you can make enough to use more than once. Here are some tasty alternatives to sandwiches:

Soup, kept warm in a flask and drunk from a cup. You can buy ready soups or make your own from leftovers.

Salads - potato salad with a little mayonnaise and some chunks of cucumber is nice, with a spoonful of tuna or some nuts for protein. Pasta salad is great too, and very filling, as is couscous.

Pies, pasties, flans or quiches - again, you can buy good ones or make some yourself. A quiche, say, can be cut into wedges with some individual wedges being frozen for later.

A lot of leftovers are nice if just heated up and kept warm in a thermal lunch box.

If you put 'packed lunch ideas' in a search engine you will find plenty of ideas. The recipes here look nice - a couple of them are for sandwiches but most are not.