The bus to Mostar from Sarajevo is relatively cheap and relatively comfortable. Once you arrive at Mostar, you can't miss staying at Hostel Majdas. For just $23 a night (for a double room), you'll feel completely at home by the welcoming staff. When you arrive, the hostel owner will treat you to tea and snacks in their beautiful garden, decorated with flags from around the world. The room was very comfortable and she even made us dinner, which all the guests ate together.
That evening, take some time to explore the historic old city of Mostar. A particularly interesting thing to note is the way the city is divided into the Christian and Muslim quarters. In "Stari Grad" you can find cheap drinks on beautiful patios, such as at Black Dog Pub, which overlooks the awe-inspiring (and historic) Stari Most, or Old Bridge.
Hostel Majdas offers something that no hostel offers: the Bata Crazy Tour. Something of a cult classic in Bosnia and beyond, the tour is unconventional. At times you're walking, hiking, or riding in the van. It lasts almost the entire day, and takes you into incredible nature sights, fascinating historical areas, and incorporates local character. It is led by the hostel owner's brother, Batha, who is as hilarious as he is knowledgeable. The price is steep- about 80 Euros- but well worth it. I won't spoil the surprises, as they are part of what makes the tour so great, but on the trip you will cry, laugh, and be in awe of Bosnia and its people. Don't forget to bring cash for stops along the way, and remember that this tour is not for the faint of heart.
After the busy tour, a night of relaxation on the patio of the Majdas Hostel is recommended, before the next bus to Split.