Haha, I'm not sure of I'd call a 3.5 gallon huge myself...its even too small for a stable cycle still...but, it is perfectly adequate as a permanent home for a single normal long-finned male betta. ;) I'm sure he'll adore his new home.
Just a tip...if you're getting a kit that comes with a filter or something, you can go ahead and ditch it. In such small tanks, the filter flow tends to bother bettas a little more so....and since its difficult to cycle such a small volume of water, a filter is pretty pointless anyway. Just keep up with a couple weekly water changes(at least 1 50% and 1 100%....though an extra 25% tossed in there might not be a bad addition either. I've always found my boys to be more active and brighter with at least a slight extra water change in the week in my smaller tanks)instead. ;)
As far as artificial decorations go....
Steer clear of plastic plants. Even the softest always run the risk of snagging and tearing fins. Go with silks. Much softer on your fish's fins and they're often much much 'fluffier', giving your betta more cover(which is something most any betta will always appreciate)and offering them something more to rest on.
Personally, I'm a big fan of Petcos silk plants....for the price, they seem to have the nicest/fluffiest selection. With a couple of their plants, it won't be hard to fill up a 3.5.....just make sure at least one plant is tall enough to reach/get close to the top of the water. ;)
Caves can be just about anything you like(thats proven aquarium safe, of course). Just feel it and make sure theres no really sharp edges that could snag your guys fins. If you want to save a little money though, this is definitely where you can do it....a little terracotta pot from the craft store(less then a buck)can make an excellent cave. Just be sure to either knock out the bottom or plug the hole with some filter sponge/aquarium sealant/make sure its well covered by substrate if you can and you're good. You can also use glass cups or even coffee mugs, just be sure to rinse very well(which is something you should do with anything you add to your tank anyway)in hot water/soak well in hot/boiling water and rinse rinse rinse if used/exposed to soaps and such.
As far as live plants and such go....well, I'm not really he best person to ask I'm afraid. I have the worst green thumb ever....I kill bamboo xP Can't manage to keep any aquatic plant alive either besides some hardy non-rooted plants.
If you really want to look into live though, a 3.5 would make an awesome nano NPT. Take a look at this thread....it'll explain things a little better: http://www.bettafish.com/showthread.php?t=114575
Also, the planted tank section of our forum in general has some awesome info. Its definitely worth going through even just some random threads and reading over what they have to say. http://www.bettafish.com/forumdisplay.php?f=147