Essentially, a "spiritual bouquet" is a gift of prayer. You are praying for the intention of your priests (that God will bless them, guide them, heal them, etc. God knows better than you, me or anyone what they need, so we leave that part up to Him). The spiritual bouquet, itself, is an abstract concept since it can be almost anything (as evidenced by the long list of suggestions). The suggested devotions are uniquely Catholic but may not be familiar to all Catholics who have not been introduced to them. All of the devotions are designed to stir up greater love and closeness to God. Let's go through the list...
Rosary: This is a devotion to honor Mary prayed on a loop of beads. It is probably the best known and most recognizable Catholic devotion.
Eucharistic Adoration: First, let us recall who the Eucharist is. When the bread and wine are consecrated, they cease to be bread and wine but are instead Jesus' body, blood, soul and divinity physically present with us. Adoration is a time when a host is placed in a monstrance (a kind of holder with a window to view its contents), usually for an hour or a day for us to worship Jesus in His physical presence.
Mass: While you know what the Mass is, did you know you can explicitly request the priest to offer the whole Mass for the intention of a person or group? Usually, this person is named in the bulletin next to the Mass times for the upcoming week. They are not always named aloud during the Mass, however.
Chaplet of Divine Mercy: As prodigalson mentioned, this is another devotion that is prayed using the rosary beads. This devotion focuses on Jesus' suffering and death and its connection to our redemption.
Novena: Mentioned by prodigalson, this is nine days of prayer for one intention. You don't have to take a vacation. Whenever/however you pray, offer it for your priests. There are many suggested novenas out there (this one looks fairly simple - just change "Pentecost Sunday" to a day that's actually ahead rather than behind us). You can probably say an Our Father each day, too (simple is good since you are new to the Spiritual Bouquet game - you can go deeper when you are ready).
Then give them a note or card that tells what you are doing. They will appreciate that you are praying for them.