9. Sandwiches

Sandwiches: a fairly easy solution (source)Cold deli sandwiches can be a relatively easy and practical idea for a strictly kosher guest. Here are some things you could get:

• A kosher-certified loaf of bread. Sometimes these are hard to find, but they do occasionally exist in mainstream grocery stores. Check with your guests about the hechsher, and leave the package unopened. If you can't find a loaf of bread that is kosher, you may have more success with bagels.
• Mayonaisse, mustard, or other condiments. Again, check with your guests about the hechsher, and do not open the containers or break the seals. Have disposable flatware on hand for serving. Squeezables make things easier.
• Some kosher deli meat in an unopened package. Once again, just because it has a hechsher, you can't assume that it will meet your guests' standards. For example, some people are unsatisfied with the commonly available Hebrew National brand, although others will think it's just fine. If you don't have access to kosher deli meat, you might have more success with cheese or fish, such as lox (smoked salmon).
• Fresh vegetables, such as onions, lettuce, tomatoes, peppers, alfalfa sprouts, or cucumbers. Ask your guest about bringing their own knife and cutting board so they can slice them when they arrive. Also, give them a chance to check them for bugs first. (Lettuce is notorious in this regard.)
• Packaged, kosher-certified toppings or sides, such as pickles or potato chips.
• Kosher-certified drinks in unopened bottles or cans. Most Coke and Pepsi products are kosher-approved, even if they do not say so on the label. Check with your guests. Also, tap water in a disposable cup is not likely to pose a problem.

To reiterate, it is not a good idea to surprise your guests with a meal like this. They would probably appreciate it the most if you coordinate with them and work out the details together.