I kept it really simple & cheap for these, all up each bag only cost me about $3.50 (which is a record for me by a long shot!!).
I was originally planning on making 'goldfish in a bag soaps', but after looking at buying all the supplies for them I realised they were going to cost way more than I had budgeted for. I really liked the look of them though so I decided to style my bags in a similar way.
I started out with clear cello bags from the dollar store ($2 for 25). I wanted the base of them to be flat, so I snipped off the bottom of them and then used this tutorial (and some washi tape) to get the shape I wanted.
Next up I found an image I liked online and digitally coloured it using fireworks. Then I printed them and cut them out.
I popped a cardboard fishbowl into the front of each bag, filled them with a couple of ballons, a mini slinky, some stickers, a fish colouring in page, colouring pencils and some sweet little lollies (I used ones that were left over from decorating which helped keep the costs down further).
Simple and cheap, and the best part was that Max loved them so much and he got such a kick out of handing them out to his little friends :).
You can see the all the photos from Max's party here and a tutorial on how I made his cake here.
Edited - Here's a link to the fishbowl image - http://stuck-on-glue.blogspot.com.au/2011/02/cut-and-color-printable-fishbowl.html