>>> If you want more water, but don't want to take up too much space, I highly recommend hitting you local homegoods store. Oftentimes, they have large glassware, that is not only unique, but I find them suitable for bettas.
Just PLEASE do remember that the more "water surface" you have, the better, so don't get something fat at the bottom and small at the top (also, terrible for cleaning, I assume). And choose wisely, taking into consideration all of your equipment.
>>> I, personally, recommend at least a few live plants. IME, bettas with live plants are more active and more colorful, which tells me that they're happier and healthier. Plus, of course, plants are good for the water chemistry... And they're pretty.
Try > Anubias
> Java Fern
And you can get a "Moss Lawn/Patch" at many pet stores, if you want one.
I'm trying to get Hairgrass to grow as a lawn, but it's not too easy. :-/
>> I try to stick to bunch plants for my smaller set-ups, because they're kind enough to grow up and around, rather than out. The exception is Java Fern, which I now have a piece of in everybody's habitat. That's how that stuff grows, FTR.