The melt and pour soap base is perfect for this. The best thing to use for soaps that you're putting things in is the
Crystal Clear soap base. It's really good about staying clear through the process though you will want to pour slowly to avoid air bubbles and such. It's really simple to work with. You can melt it using a double boiler method, but honestly, I just cut it up into cubes and stick it into my huge Pampered Chef glass mixing bowl and zap it in the microwave. You need to do it in intervals and stir in between... kind of like melting chocolate.
That website also has a video on embedding things into soap like this at:
http://www.wholesalesuppliesplus.com/VideoPlayer.aspx?VideoName=Melt_Pour_Soap_Embedding Included is a cute demonstration for the rubber ducky that shows you how to do easy "bubbles" for the bath he's taking.
Again, not associated with the site in any way, I just found them to be a great resource for information when I was getting started. There are also forums there where you can post questions, etc.
As far as curing time goes, the melt and pour only needs to stay in the mold long enough to set up solid - a few hours - and then I unmold it and leave it on a paper towel on the counter overnight. With the goldfish in a bag soaps, just leave the bag top overnight before closing up with ribbon or whatever you're using.
Good luck! I find making cute melt and pour soaps like this loads of fun