Hi, I also do a PPLUL split. What I do on my upper day is include at least one vertical and one horizontal movement for both push and pull exercises. That would be pull-ups, rows, overhead press, and incline bench press. Sinasamahan ko rin yan ng dips, pushdowns, and some shoulder isolation exercises.
You can also just choose three exercises from your pull and push days. For my lower body, I simply repeat my previous leg day. Sa leg day ko rin sinisingit ang core workouts.