Ross getting hit in the back by a water balloon in Elizabeth's apartment was unintentional and as such his reaction was ad-libbed.
Dr. Richard Burke (Tom Selleck), recites an excerpt from a poem at dinner: "In the sweetness of friendship, let there be laughter and sharing of pleasures. For in the dew of little things, the heart finds its morning and is refreshed." It is the end of Kahlil Gibran's "On Friendship".
Monica encourages Ross to make a list of pros and cons about his relationship with Elizabeth. Chandler and Joey did the same thing with Ross about Rachel and Julie. The first time it ended with disastrous results. It's surprising that Ross would risk that again.
The jokes between Ross & Rachel about him buying lots of engagement rings and her not getting one was originally written for the previous episode but edited out due to time constraints. It can be seen in both episodes on the longer DVD versions of the episodes.
The crystal on the table at the restaurant where Monica and Chandler are having dinner is Baccarat (or they're good replicas). The champagne is Dom Perignon. It really IS a fancy restaurant.