Invitations do far more than inform guests of the event they are attending. Invitations create an expectation, and an understanding of what their experience is going to be catered to.
Visuals on the invitation
Remember that the design should fit the occasion, while still maintaining the intended tone of the event.
Visuals can amplify that set expectation to provide a little more detail on what activities will be available at the event, expected attire, etc. This can be in the form of photographs, custom graphics, or expressive text fonts.
Keep your target market audience in mind
If the event is a baby shower or bachelor/bachelorette party, the invitations can be less formal. This separation intended to allow the host to personalize the invitations and focus more on immediate family and friends.
Weddings and other events with high guest volume will need to allow more space for general information on the venue, catering dietary options, and other more acute details.
Say more with less on the page
In the same breath of ensuring the details of the event are included, remember that simple is sometimes better. Be sure to present the necessary information for the guest to know quickly and aptly, avoiding dense text blocks or fonts that are difficult for the average eye to read.
Digital invitations vs mailers
Mailing invitations can create a more personalized appeal for the recipient, as physical print can feel more inclusive for most guests. However, the digital space is a bit simpler to send out a quick blast to the email addresses of a higher volume of guests.
The recommended approach is to cover both bases, and pair them together with a similar visual and graphic design. This will create an inherent reminder for the guest to appreciate the physical invitation, and be more likely to respond to the email invitation later.