Summer picture courtesy of PPJ (thanks!)
Rundtjern was my (Ainas) first childhood trip destination when I started walking alone in the forest, and later it became the first 'check-point' on trips in Østmarka. Easiest access from Haugerud, Oppsal or Østmarksetra. Rundtjern is close to Lutvann (be aware steep cliffs), and you can follow many nice paths around these lakes. Going to the cache, you will experience typical Østmarka terrain - smoothe flat hills going north/south, hilly with steep cliffs and constantly up&down walking east/west.
This part of Østmarka has many miles of illuminated tracks and 'hundreds' of paths criss-crossing the forest. I recommend an evening trip in the illuminated tracks to Rundtjern - very 'trolsk' - but you will probably need to visit the cache before dark (or bring a torch/maglite for the final strech).
The cache is a black lock-and-lock box, containing a log book, a pencil, a pen and the Geocache info sheet.