Every ancient coin is associated with a place, at the least where it was minted. Rarely, but occasionally, we learn where a coin was found. Many ancient coins depict the personification of a nation, province, city, or river. Every coin has some tie to geography. Of course collecting every coin is not a theme, so geography must be narrowed down in some way. Collecting the coins of one mint, city or region is popular. Hadrian's famous "travel series" would make an excellent geography theme collection. Another is the travels of Paul. Or you could collect coins from all places you simply find captivating.