Line a baking sheet with waxed paper. Make trees: Trim long side of rice cereal treats to form triangles. Make an indent at flat end of each treat to insert a pretzel.
Dip tips of 4 pretzels into green melts; insert each into a rice cereal treat. Place on baking sheet; chill until set, 5 minutes. Dip a cereal treat into green melts to coat. Let excess drip off; place on baking sheet and chill 5 minutes. Dip remaining pretzels in pink melts to coat 3 inches of end. Transfer to baking sheet; chill 5 minutes.
Spoon melted white and dark cocoa melts into separate ziplock bags; seal bags. Snip small corners off bags. Pipe a drizzle of dark cocoa on each pretzel near base of tree. Pipe loops of white melts over trees; sprinkle with yellow sugar. Pipe dots of melted candy and attach M&M's Minis as lights. Press spice-drop stars on top of trees. Chill until set, about 5 minutes.
Make Santas: Use icing to attach a cut fruit slice at top of pretzel as hat. Pipe white melts; attach cut marshmallows as hat trim, beard and mustache. Attach mini chocolate chips as eyes; cut red M&M's Minis in half and attach as nose. Attach pink heart décors as mouth. Chill until set, 5 minutes.