I really like day-trips, especially cross-border ones. So, Chad+Cameroon is on my list.

I did:
Strasbourg to Kehl (With a tram and on foot)
Tallinn to Helsinki
Vienna to Bratislava (Twice)
Vienna to Brno
Zagreb-Maribor-Vienna (It was when I was returning after spending a few days in Zagreb, and spent a few hours in Maribor)