I mean to prompt specifically about drinks or desserts by asking people to add them?
The best way to up-sell is to make a promo deal with the things you want to promote and people would really buy it more. As part of a promo deal we also offer the ability to highlight deals contextually, if a food client did not choose to chase that deal from the start.
The “kind reminder” made outside the context of a promos deal does not have the same impact so we do not recommend nor support them because having too many UX diversions like this, without an actual benefit for the client, will just confuse, defocus and hassle the food client, but not really make any point and have little to no impact comparing with promoting a true value combo or some other promo deal.
If you still want to try some “light upsell” technique without actually granting any benefit to the food client, you may play with the categories order in the menu.
For example this restaurant sell a lot of promo deals and to increase even further their upselling on drinks and desserts they just put those categories first in the menu.
It worked like a charm from them -> less noise but more results.
Another way yo do it is to set the “add a cake or drink?” question as a mandatory add on where people are forced to either choose one of the proposed options or choose the “no thanks” option. This way you get exactly what you are looking for but it may hassle and irritate the clients so we’re not really recommending this but you may try it…