"Contemplata aliis tradere." In the felicitous phrase of St. Thomas, Dominicans are to "give to others the fruits of their contemplation." If you don't do both, the contemplating and the giving, you aren't following the way of Saint Dominic.
How do Dominicans, and Lay Dominicans in particular, contemplate?
Oh, I don't know, you know, don't you know. The only description I've heard of a specifically Dominican method of contemplation is given here. For all the theologizing Dominicans do, we seem to be a bit loosey goosey on the theology of contemplation, and when I have seen Dominican theologians discuss it, they're usually quoting St. John of the Cross.
Still, for the most part, when Dominicans speak of contemplation, they mean contemplation of the Truth Who is God. This is done by study, but chiefly by prayer. Explicitly called out in the Rule for Lay Dominicans are daily Mass ("as far as possible"), the Liturgy of the Hours, the Rosary, and periodic spiritual retreats.
None of these is distinctive to the Order of Preachers. Dominicans contemplate the way Catholic contemplate, and different Dominicans will contemplate in different ways.
But we must always balance the contemplative with the apostolic. We are an order of preachers. It's not enough to gather fruit, we must also go forth and give it to others.
This may be a bit controversial, but: Let me emphasize the "go forth." The word "apostle" means "one who is sent." Hospitality, the habit of sharing what you have with those who come to you, is a wonderful virtue, but it is in a sense a passive virtue. To preach is to share what you have with those to whom you come, to actively seek those who are in need of what you have been given to share.
Ordained friars, I've always felt, have it easy. They can preach daily at Mass. The Eucharist brings the crowd, and good manners keeps them there till the friar is finished.
For Lay Dominicans, apostolic activity has to take a different form. (And of course liturgical preaching isn't the only apostolic activity for ordained Dominicans, either.) The Rule calls on them to
bear witness above all to their own faith, listen to the needs of their contemporaries, and serve the truth
in a special way to show real compassion to all who are troubled, to defend liberty and to promote justice and peace.
The specific activities undertaken are determined by the individual Lay Dominican, coordinated as appropriate with other members of their chapter and other individuals and apostolic associations.
What these activities are, Lay Dominicans are free to decide. That such activities occur, Lay Dominicans are bound by their promises to ensure.