All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have already issued many Fataawa on what to do when blessed with a new born baby and we clarified the recommended acts which a Muslim may do on this occasion, for instance you may refer to Fatwa 82088 in this regard.
As regards a marriage ceremony, it is permissible for women to apply the Duff (a tambourine-like instrument without bells) as a sign of joy on this occasion, and recite poetry provided their voices are not heard by men, as this causes temptation. Allaah Says (which means): {…do not be soft in speech [to men], lest he in whose heart is disease should covet,...}[Quran 33:32]. The evidence about the permissibility of what we mentioned is that the Prophet said: "The difference between what is permissible and what is impermissible (in the relation between a man and a woman) is the sounding of the Daff and the voices of women in marriage ceremonies." [Ahmad and others] However, it is not permissible to use musical instruments other than the Daff as there are many narrations which prohibit the use of musical instruments which also include musical sounds generated by computers or other electronic devices.
To conclude, it is permissible to celebrate the marriage ceremony and gather in order to announce it on this occasion, provided that this is free from prohibitions and transgressions of the religious limits, like songs accompanied by musical instruments, dancing, mixing of men and women, revealing the 'Awrah (parts of the body that must be covered in Islam for both men and women alike), and so forth; so if the ceremony is free from all of this, then there is no harm in it.
As regards your statement "without going out of Islam" if you mean abiding by the religious limits, then the religious limits have already been discussed. But if you mean to go out of the religion of Islam completely, then the ceremonies, even if they contain some prohibitions, they do not take the doer out of the fold of Islam, as these are acts of disobedience, provided they do not reach the level of apostasy.
Finally, one should avoid invented ceremonies which are not approved by Islam, like celebrating the New Year, or birthdays, and the like.
Allaah Knows best.