Is it ok to show up to a conference without registering if I don't eat any meals? I'm planning to attend a prestigious conference in computer science. The registration fees are very high (~150$ per day). I'm presenting as well, but on a co-located event directly after the main conference.

I don't know whether my university will fund my registration fee for the whole conference. They will most certainly fund the fee for the little event where I'm presenting. In case they don't fund the whole conference, is it acceptable to show up to the conference without being registered? (Of course, I wouldn't have the conference meals with the other participants on the days I'm not registered.)
No, it is not acceptable.

You can of course email the organisers and ask if you can participate without paying, but do not be surprised if they say no.

(Please note that conference registration fees cover lots of things besides the lunch. Among others, conference registration fees may cover the rent of the hall in which you are sitting, and expenses related to the speaker who is giving the talk. Conference organisers are definitely not expecting random people to come there without registering, unless they explicitly advertise this possibility.)