According to the 1983 Code of Canon Law:

“A deacon can impart only those blessings expressly permitted by law” (CIC 1169 §3).

Which blessings are permitted to deacons?

We find that the deacon may perform the nuptial blessing in Holy Matrimony, and may bless holy water in baptism. He may also perform blessings in the funeral rites. The Book of Blessings (published by Congregation for Divine Worship) decrees that deacons may bless certain sacramentals. The Congregation for the Divine Worship lists out those sacramentals that a deacon may bless:

“medals, small crucifixes statues or pictures that will be displayed elsewhere than in a church or chapel, scapulars, rosaries, or other articles used in religious devotions” (BOB §1442).

Elsewhere it particularly states that deacons may bless rosaries (BOB §1465). Deacons may also bless holy water, but only outside the context of Holy Mass (BOB §1390).

Deacons may also bless the private home of a Christian (BOB §661). However, deacons are prohibited from blessing large images of Christ, the Blessed Virgin, or those of the saints (BOB §1261) that shall be displayed in churches or chapels. Further, only priests or bishops may bless things like confessionals (BOB §1205) and pipe organs (BOB §1326). Bishops reserve the proper right to bless cathedrals and sacred chrism.