Buy for the facilities you have
Check whether the accommodation has a usable refrigerator, utensils and any cooking equipment. A minibar is not necessarily suitable storage for a week's groceries, and a kettle does not make a room a kitchen.
Choose quantities you can finish during the stay. A large package may have a lower unit price but still waste money if most is left behind. For a short visit, a simple breakfast or one picnic can be enough to make groceries worthwhile without reorganizing every meal.
Put the shop on the route
Find a store near the lodging or the day's walk rather than making a separate cross-city trip. Check opening hours for the day you arrive, especially on holidays or a late evening. Do not assume an airport or station shop will provide the same selection or prices.
Keep the purchase small enough to carry comfortably. A heavy bag of food can make a museum visit awkward, and many attractions restrict eating or large bags. Buy close to the time you will use the food instead of carrying it through a long sightseeing day.
Choose the eating place before the menu
Look for a suitable public space with seating and check its rules. Paris's municipal park information, for example, covers opening hours and permitted uses. A lawn may be closed, a garden may shut early or a space may have restrictions on gatherings.
Plan a rain alternative that does not involve eating where it is prohibited. Keep waste contained and leave the place clean. Avoid food that needs temperature control unless you can keep it appropriately chilled; buying a simple meal shortly before eating is often easier than trying to transport it all day.
Compare the whole purchase
Include drinks, utensils or storage items you actually need. A picnic assembled from many specialty items can cost as much as a modest café meal, which may still be perfectly worthwhile if that is the experience you want.
Groceries are an option, not a rule for every day. Use them where they fit the route and facilities, then leave room for local meals that interest you. The saving comes from a usable plan and sensible quantities, not merely from choosing a supermarket over a restaurant.
