I was only there for 11/2 days so only two walks. For the first I caught the cable car up to Hirschenkogel (1340m) and went up the observation tower. Then I followed a made-up path to Sonnwendstein (1500m). This is quite a pull but there is a restaurant at the top. After refreshing yourself make sure you do the last 50m up to the church and vista point.
The following day I did the Bahnvanderweg which follows the railway line back from Semmering to Breitenstein. This is a lovely walk and is mostly gentle because it follows the contours of the railway. Just a few up and down bits. Its 9.5 Km but you can go even further if you wish.
There is not much to Semmering itself except for the beautiful location and the Panhans Hotel.