Most plants will do fine in just gravel. I only have about 1cm of black gravel in my 3.5 (to save swim-space), and though most of my plants are not soil-rooted (java fern, susswassertang, potted crypts & anubias, java moss) the ones that are (wisteria) are doing just fine.
You may need to dose with liquid ferts (don't use Flourish Excel, I've heard bad things about it with fish in smaller tanks..regular Flourish is great, though) now and then and maybe some roots tabs for the root feeders (like swords and lilies), if you don't have soil for them to feed on. Some people dose a drop or two of Flourish every day, so it's not harmful to the fish. That won't help the root-feeders, though, so tabs are best for those.
If you don't wish to do ferts or soil, I've found java fern tied to a bit of wood not only looks wonderful but grows like mad just on fish poo. Java moss in clumps looks great, suswassertang too. Anubias can be tied to anything and be happy feedig off whatever's in the water column.. I've seen some tanks planted quite heavily just with various varieties of those few plants and they looked beautiful.
Then there's the floaters.. floating plants are awesome and the bettas love the 'shade' they provide. Frogbit, duck weed (if you don't mind scraping it off your arm/equipment every time you touch the tank) etc.. and they don't need ferts either.