I would say add some gravel, the ornament, and as much as you can if you really want to speed up the cycle. It is only temporary so the small castle would only be in there for seeding purposes and then removed. I do think it should be removed as soon as the tank is established or around 2 week into it as it won't serve much purpose after that. If you can, take an actual piece of the filter floss or media from the established tanks filter and place it directly in the filter for the new tank. You could place a sponge filter or a piece of filter floss in the old filter or in the gravel so it can become cultured with some bacteria and then put that in the new tank also. Lots of options and the more you get in there from an eatablished tank the better. If your LFS will do it, get some from them also.
As for fish, neons would be cool and fun to watch, glolite tetras, (especially the albinos
) and some bottom feeders like cories would do well. I have seen a fish but I haven't been back to the LFS to find out what it is that feeds on the surface, almost looks like a guppy with a turned up mouth but is a really cool fish. Hopefully someone will tell you what it is as I have yet to find it online. Cories like to play in caves and such and a few pieces of driftwood or bogwood would be a nice addition for them. Skunk botia loaches or other smaller loaches are a possible choice also but I wouldn't recommend them with cories or other bottom dwelling hiding fish. Mystery snails would be ok and you can keep them from reproducing by either keeping one or removing the eggs that they lay above the waterline. Might be something for a school project to take the eggs to school and let them hatch. Will try to find that fish unless someone beats me to it.
Lots of options, if you are unsure about a fish, just run it by us in here and we will help.