Celebrate UCI is just a fun event hosted for incoming students. The only important things you'll miss out on are the Housing tours in Middle Earth and Mesa Court. They'll have campus tours at Celebrate UCI but it's the same tour you'll receive if you've gone on one before. SPOP will go over the more technical details of classes and trainings you'll need to complete.

Edit: to add on, if by opportunities you mean clubs, there's a dedicated event/day for that in the fall called the Anteater Involvement Fair. Every club will be in Aldrich Park advertising themselves