Buses and minivans are the major means of transportation within the country. From Yerevan you can get to literally every place in Armenia within a day. To make things confusing for foreigners, there are several different regional bus-stations in Yerevan and the minivans tend to leave from hard to find places just somewhere at the side of the road. When heading into Yerevan, they are not unknown to drop you at random spots somewhere in the city, so ask the driver beforehand to drop you at a convenient place. The following is a incomplete list of the major bus stations.
There are many convenient day trips based from Yerevan, including a bunch better saved as overnight trips.
Garni and Geghard are an absolute must, or people will not believe you've been to Armenia.
Zvartnots Cathedral ruins and Echmiadzin.
Saghmosavank and Hovhannavank monasteries overlooking the Kasagh River Canyon and the churches of Ashtarak.
Khor Virab Monastery, Noravank Monastery, Noravank Canyon and Areni Wine Country.
Lake Sevan, Sevanavank and Hayravank Monasteries, and Noratus Khachkar Cemetary.
Dilijan old town, Haghartsin and Goshavank Monasteries.
The Debed Canyon and the monasteries of Lori UNESCO World Heritage sites of Haghpat and Sanahin, plus Odzun, Kobayr, Horomayri and Surp Grigori.