We did several different things when I was an undergrad. The standard formula though, was flowers and balloons along with the bid card and then taking them to either a lunch (indoors if it was cold out) or a barbeque/cookout at a park right next to campus. It was always really relaxed and good fun, plus at the park we could use the playground equipment and who can resist a swingset?
There were also several years where we serenaded our new members with fun Sigma songs like "1492", which was a chapter favourite.
Sigma Love and Mine,
Laura