Do you want something that will harden and therefore suspend the items in place?
Or do you want something that will just be thick-ish or rather-thick so the items can move around if the liquid is disturbed?
Here's some info from my files about both:
LIQUID-Y + GELS:
The first possibility would be glycerin or glycerin + water. That's what's usually used in "snow globes" to make the snow fall more slowly.
If you want to read more about those possibilities, and the problems that can happen with mostly-water on dark polymer clay items in water/snow globes, check out this page at my site:http://glassattic.com/polymer/outdoor_snowglobes_fountains.htm
(click on some of the subcategories under "Snowglobes")
Other people have used mineral oil, clear cooking oil, corn syrup, liquid soaps, etc. Some have even used personal lubricants.
The next possibility would be a real gel like Dippity Do. That's often used by crafters in small bottles when suspending items.
Actual 2-part resins** can be used (there are several types) but they'll be more expensive, more fiddly to use and to avoid bubbles, etc. The most common type that gets really hard would be Envirotex Lite, or one like Easy Cast.
Two-part epoxy glues would be the same thing but thicker and come in smaller amounts (the clearest brand I know of is Devcon's 2 Ton 30-Min Set).
You could also use a "softer-setting" 2 part epoxy resin like the kind that's sold for use in clear vases for artificial flowers called "floral setting resin" (there are different brand names). Those would be sold in the floral dept at Michaels/etc, or probably in floral shops.
You could also try something like polyurethane (hardware store), or acrylic "finishes," or clear acrylic fingernail polish, or even floor polishes like Future (now called Pledge with Future Shine) or Mop n Glo, or acrylic medium.
(Diamond Glaze isn't a "resin"...it's a permanent white glue that is clear when reasonably thin like all white glues, but also will dry a bit dimensional.)