you'd need some silicone, but buy a bag of river rocks, or the glass marbles (some are flat some are spherical), like for a flower arrangement and things like that. glue them all into a cave! i also use glass candle holders. Michael's has dollar bin specials a lot, and gets cute little glass candle holders, and sometimes other obscure things you may be able to use. i have two candle holders in Zeddie's tank, and three in the big tank (only one has my sword planted in it). They kinda look rustic now, with all the sand on them.
OH ALSO the glass vases/cups like Jen mentioned. Some people i've seen use the giant margarita and wine glasses in their tanks
oh there go the cories again. love those things.