I spent the last summer in Ghana, and while there I noticed how much of the food is made with New World imports. Food must have been quite different before the Columbian Exchange.
Some examples:
Peanuts (also called groundnuts) are the main source of protein.
Maize is the base starch in much of the food.
Hot peppers are used to spice everything.
Tomatoes go in every dish, most famously, Jollof rice.
Many of the uses of New World foods in West Africa are as substitutions for similar ingredients (done because the New World version was easier to grow and/or provided higher yields). For example:
Peanut: The peanut, depending on the dish, works as a replacement for (a) the bambara nut (Vigna subterranea), a groundnut similar to the peanut, and (b) sesame (benne), which was and still is used ground in soups/stews as a thickener and source of fat and protein (both sesame and peanut are about 50% fat, and sesame is 18% protein which is not too far below peanut at 26%). The bambara nut is still grown, sometimes inter-cropped with peanut (which is an excellent strategy for ensuring that not all of your crop is destroyed by pests or diseases). To go pre-Columbian style for soups with ground peanut, use sesame instead. For other uses, bambara nuts will be the appropriate substitute.
Maize: largely displaced millet and sorghum. To go pre-Columbian, make millet/soghum porridge instead of maize porridge.
Cassava: partly displaced other root crops such as yam and taro (cocoyam) (and plantain, while not a root crop, but often performing the same role, was also partly displaced). To go pre-Columbian, use yams, taro, plantain instead of cassava (e.g., for fufu). Note that some taro is New World; there was an exchange of taro varieties, and both New and Old World varieties ended up being grown on both sides of the Atlantic.
Beans: New World beans, primarily Phaseolus vulgaris, partly displaced Old World beans, notably the black-eyed bean (black-eyed pea), Vigna unguiculata. Black-eyes are still widely used in West Africa, and can be substituted for P. vulgaris (e.g., kidney beans) for pre-Columbian-ness.
Chillies (hot pepper): have partly displaced other "hot" spices. Pre-Columbian hot spices include Aframomum melegueta (grains of paradise, melegueta pepper), Aframomum citratum and A. danielli (mbongo pepper, alligator pepper, Guinea pepper), Xylopia aethiopica (Senegal pepper, grains of Selim, Guinea pepper, and various other names), Piper guineense (false cubeb pepper, Ashanti pepper). And ginger, if you consider that a "hot" spice.
Tomatoes are different from the above, in that the appear to be an addition, a new ingredient that doesn't just (partly) displace an old ingredient, but is completely new. My only suggestion is that they must simply be omitted to go pre-Columbian. Note that many dishes will still be conspicuously red from the use of palm oil.
Some traditional recipes are very simple to make in a pre-Columbian manner, and can be used as likely examples of pre-Columbian West African dishes. Egusi soup, with chillies replaced by pre-Columbian spices (and pumpkin leaves by other greens, if the recipe uses them), is an obvious choice. Rice and beans, with tomatoes omitted and chillies substituted or omitted, and New World beans replaced by black-eyes, is another. Sesame/benne/beniseed soup, tomatoes and chillies as above, another. Of course, replacing modernisms like stock cubes with traditional versions such as dried/fermented seafood.
For a good overview of the evolution of African cuisine, including the impact of the Columbian exchange, see James C. McCann, Stirring the Pot: A History of African Cuisine, Ohio University Press, 2009.