Adama has a better restaurant scene than its size would suggest, and the reason is straightforward: it sits on the busiest road in the country, and truck drivers, traders, and transit passengers have been feeding a competitive restaurant trade for decades. The food is solidly Ethiopian — injera with a range of wots (stews), tibs (pan-fried or grilled meat with rosemary and green chilli), and kitfo (finely minced raw beef seasoned with spiced butter and mitmita). A proper kitfo, made from good highland beef and eaten at room temperature rather than cooked through, is one of the best things you can eat in Ethiopia, and Adama does it well.
Oromia is prime coffee country, and the coffee ceremony — green beans roasted over charcoal, ground by hand, and brewed three rounds in a clay pot — is taken seriously here. Almost any restaurant or café will offer it if you ask, and many run the full ceremony in the afternoon. The coffee itself tends to be excellent: single-origin Harar or Sidama beans, prepared without sugar unless you specifically request it with. If you are not used to Ethiopian coffee, the first tiny cup will surprise you with its intensity.
The main restaurant strip runs along the central boulevard near the bus station. Look for places that are busy with locals rather than emptied-out establishments catering to truck stops — the quality gap is significant. Most of the better spots are open from early morning for breakfast (firfir and tea) through to late evening.