Wide Open Spaces in Home

Define giant. Is the space bigger than your living room area? If yes, you could probably make another room there. If you like to have a huge foyer then you could use it as occasional party space. Put a piano in a corner and maybe a table with a huge vase of flowers in the center. Make sure the floor is laid with marble so it makes a click-clack sound when the sims walk there.

Bookshelves
Aquariums
The games table
Umbrella stand
Cabinets
Plants

Just a few suggestions. :)

If it were mine, I'd probably put a fountain or large sculpture in the center. Then I'd add a couple of benches, place some strategic clusters of plants (some large, some smaller, but no tiny ones), and leave the remaining space open. I'd be going for something like an indoor mini-park or garden or atrium-like feel.

A large open foyer demands a decorative round table smack dab in the middle of it (and a round rug underneath) with a big beautiful flower arrangement sitting on the table. :)

depends on a shape of space like can i cut additional room into it or would that look silly

if its ridiculous big i would probably use it as dance /party hall

I don't have this problem because I hate overly large houses. And entryways/hallways in my sims homes is not necessary for me personally, it just adds more headache of walls blocking my view or routing issues.

I have a tendency to have my front doors to open directly into the main living room with a little extra "walkway" space. Or if I do have some sort of entry I use a small 2-tile wall to break it up, or even small short walls. I also usually create open-living spaces so my living room/dining room and kitchen are pretty much in the same room area with very little walls that sims need to traverse through doorways.

And, also 90% of the time I only play in single story homes, even if it's a 2-story or larger home from the outside, I make sure there are bedrooms on the main floor so my sims can live on one floor and I delete all of the stuff from the upper level rooms (typically deleting the walls and doors but leaving the stairs so it appears to be a proper 2-story home. This way I still get the aesthetics of a nice 2-story home from the neighborhood view but I have better control of keeping an eye on my sims.

And finally I always play with walls up because I like to see the deco on the walls and when I play with walls down all of the wall items distracts me because it's just a big jumble of floating stuff.

So yeah, large houses for me doesn't fit my personal play style.