There is not another suitable place to have lunch en route from Sofia to Belogradchik apart from Hotel RIU Pravets where they would have lunch on the previous day. I recommend to keep the lunch in Sofia. We can include also visit to Boyana church this morning and make the city tour between 09:00 and 12:30. Lunch in Sofia between 12.30 and 13.30 and then depart to Belogradchik. Arrive in Belogradchik at around 16.30-17.00. The fortress is open until 18:00. Last entrance is at 17:00. So, depending on time of arrival they can make it either on this day or in the morning of the next day.