I'm looking forward to walking around the Izu Peninsula with a few friends this summer break.
Literal
I [topic-は] this summer-break [in-に] several [of-の] friends [with-と], Izu Peninsula [object-を] walk-and circle-once-do [nominalizer-の-を] looking-forward-to-am.
楽しみにする/楽しみにしている is the standard 'looking forward to' expression — 楽しみ ('pleasure, anticipation') becomes the state to be in via にする. The nominalized clause 歩いて一周するの ('walking around once') becomes the object of 楽しみにする. 歩いて一周する uses the te-form to chain 'walk + circle once' as a single sequenced action.