One of the best birthday ideas for a five year old girl is to come up with a themed birthday party or a dress up party. You can come up with a little mermaid theme and buy theme decorations to match the party theme. Prepare some fancy pink drinks and maybe throw in a candy buffet.
Orrrrrr, how about cake, ice cream, and little friends?????? Get her involved in decorating. Check out Pinterest for clever Inexpensuve ideas. Or even if you do have a budget for such things, how about just celebrate the miracle of birth and let her know she's special and loved, not the Princess of consumerism. Just a thought.