For my 7 year old fishy theme party I am making invitations in an off white teared edges placed in a small plastic bottle with a sticker of a fish on it. The cake is round shape icing is blue with a long black licorice with a gummy worm at the end  ,using wafer candy arrange and shape it into a fish near the hook, I will have a bowl filled with blue raspberry Jell-O with gummy fishes. I will also serve golden fish crackers. Also served will be tuna fish sandwiches. As favors I will be giving fish theme toys . Activities we will make our little fish tanks by using small jars(baby food jars will be great) fill jar with water and blue food dye -hot glue little stones or gravel even glitter on the jar lid (underside). add a small plastic fish in water close the lid . It quick and they don't have to worry about parents taking care of a live fish and it some thing enjoyable. Other games are guessing games where we put sea theme toys ,shells, plastic worms in a bag and the kids have to guess what is in it . Hope these ideas come in helpful . My daughter loves fish she made special certificates for her friends with fish on them as thank you for her friends.