Sounds like you're off to a decent start. Can you share for us the wattage of your heater and the make/model of the filter? I assume they'll be good since you say it was a kit, but just to be safe, I'd like to know.
This is gonna get long, but I promise it's thorough and important. Grab a seat and hold on.
One of the first things I'd check are the parameters of your tap water (or whatever source you happen to be using). If you have soft, acidic water you have different fish options than if you have hard, basic water. This is as good a reason as any to procure your API Master Test Kit that will let you check all the parameters you could ever need to monitor about your tank. It will also make the cycling process way easier. Read all about cycling here
It looks like you have a pretty solid idea of what you're wanting. Substrate choices are as varied as the sun and (disregarding for a moment the preferences of some fish species) mostly up to you. Some swear by gravel, others sand, and even some bare bottom tanks. You'll get just as many positions on light vs. dark. Me? I'm a dark gravel girl. But you're family can be into light sand and it will work just as well. On to decorations.
Decorations conservatively include plants (fake and live), rocks, and wood. Yeah, there are Roman Colosseums, Sunken Ships, and such if you're into that too. Important things to know about these:
1) Live plants aren't hard. Check out our super Moderator Byron's treatise on low tech planted tanks here
. That's just part one. Parts 2 - 4 are also stickyed under the plants forum. The basics are these. Different plants have different requirements including light, ferts, and planting location. Some like to be planted on wood or rock, some float, and some like to be planted in the substrate. Do your research, but good low to moderate light plants to start off with include Java Fern, Anubias, Amazon Swords, and Banana Plants.
2) Rocks rock! I'd stay away from the lava rock you mention though. Unless I'm mistaken (someone please chime in if I'm mistaken) the have a chemical makeup that's not the best for aquariums. You want to avoid calcerous rocks. That is, if they bubble under a strong acid, stay away. That'll be stuff like limestone. I believe Nitrate test bottle #2 in the API Kit I mentioned earlier will be strong enough to help you sort out the good from the bad. This also means you can go collecting from the "wild". Just make sure they're clean (rub them down with water to get rid of any dirt etc) before they go in the tank. And DON'T boil them to sterilize them before you put them in. There are potential explosions to be had there.
3) Wood. I tend to heavily favor wood over rock in my tanks. It's just a personal preference. Some fish have theirs too. Wood can be found fairly cheaply at any well stocked LFS (Local Fish Store) though again, you're welcome to go collecting. Again, clean it before you put it in the tank with regular old water. Biggest thing to note here is that most wood will release tannins that can temporarily play with the clarity of your water and over time will lower the pH in your tank. This is great if you have fish that like a mildly acidic water. Not so much if they like the basic.
Fish choices are also varied. Not knowing the parameters of your water, I can only offer general advice but here goes. You look to be aiming for a "community" set up with many small fish or perhaps many small fish with one or two "display specimens" that are larger/more unusual. A 36g Bowfront gives you some room to work with but there are still plenty of fish that this tank would be too small for. PLEASE try to avoid getting anything that will be cramped in this tank even if it's only a juvenile for now, for the fish's sake. You'll generally be looking at fish belonging to the livebearers (mostly basic water), Cyprinids (mostly acidic), Characins (mostly acidic), and small Catfish groups. Check these out under "Tropical Fish Profiles" in the blue bar at the top of every page. These groups will include some fish you've heard of before and many you haven't. Tetras, Barbs, and Loaches are all there along with lots of other options. Not every fish in those groups will work, but many will. If you find your family wanting a "specimen" or two as I mentioned above, check out the Cichlid group as well. Many of the smaller species there work great in this capacity. Also worth noting at this juncture is that many of the species in these groups are "shoaling fish". That is, they really only feel comfortable in groups (usually very large in the wild). Generally, for these fish you're looking at a minimum group size of six. Some have a slightly larger size than that. This need will be covered in detail on most of the profiles of any fish you'd happen to be interested in.
Now for the fish I'd avoid no matter what they tell you at PetCo or wherever you end up going to buy your stock.
Bala Sharks, Tinfoil Barbs, Tiger Barbs, "Sharks" of all varieties, Discus, Angelfish, Goldfish, anything that looks like an eel or ropefish, Bettas, Common Plecos, Siamese or Chinese Algae Eaters, anything that looks like a manta or stingray. This is a long, but by no means exhaustive list. If you had your heart set on anything on that list I'll be happy to explain my reasoning. The good news is, the list of fish that WILL work for your needs is even bigger. Go do some more research, get your water checked, start making some tentative decisions (one big school, a couple smaller ones, etc.) and ask questions here as long as you like and as often as you feel the need.
Sorry to be so longwinded, but better to be informed from the start. Good luck!