By busBuses go between depart from Plovdiv's "Rhadopi" bus station in the south of the city at least twice an hour normally, mostly on route to Smolyan or Kurdzhali. See Bulgaria#By bus 2 for approximate timetables.
The Castle of Asen, also known as Asenovgrad Krepost, is a medieval fortress in the Bulgarian Rhodope Mountains, 2–3 km south of the town of Asenovgrad proper, on a high rocky ridge on the left bank of the Asenitsa River. Very worth seeing although a bit hard to access. There is no evidence of public transit and it requires a 30-40 minute walk from the city proper. Admission is free.
Bachkovo Monastery is the second largest monastery in Bulgaria after Rila Monastery. Significantly more developed that Rila, it has less of a serine atmosphere, however it is very beautiful and is much worth a visit. Getting there is extremely easy, as buses on the route Plovdiv-Smolyan stop in both Asenovgrad and Bachkovo. Tell the driver you are going to the monastery (simply saying "Bachkovo Manastir" should do the trick) and he may drop you off right in front. As of September 2012 the journey cost 2 Leva.