I was PR chair for my chapter 4 years in a row, plus Panhellenic so I have few ideas to pass on...
Of course as you have mentioned letters days are a great way to get your name out.
Create a website if your chapter doesn't already have one
do small on-campus philanthropy events such as bake sales, penny drives, clothing drives etc.
participate in intramural sports - great way to meet lots of new girls
have sisters join other campus clubs - also an awesome way to meet new people
publish accomplishments in school paper
participate in campus events such as 1st year orientation day
participate in greek events
have a pr competition in your chapter - this worked really well: each month hand out a pr star certificate (to the sister with the most pr points for that month) and at the end of the year hand out a certificate to the the PR superstar (the sister who has accumulated the most points over the school year) Each week I handed around a spreadsheet during meeting for sisters to fill out - I had different catagories: service, sisterhood, social, sports, wearing pin, wearing letters - each category would be worth a certain amount of points - the sister would fill in how many days or times she did these things. I also handed out an award at the end of the year to the team that had the most points.
I hope this helps
