Need ideas for inside fireplace

Do you mean a non-working fireplace? Some add a log rack and some nice white birch logs stacked. Some add a multiple candle rack with white candles especially the luminaires you can turn on. I have seen some make a fireplace screen from plywood and decorative paint it, this real popular in country styles.

Do you already have the fireplace or are you wanting to put one in?

Nice Vase or Jug of Flowers/Foliage with the seasons (Fresh or Artificial)or Mix.

Large Ornament -Electric Fire of Logs or Coals - A Display of Photo 's -Painting - Mirror etc.............

Use large pillar candles in different heights. Looks great when lit!

I used a big piece of driftwood I found on the beach along with a fire place candleholder and an arrangement of shells. You can buy very cool summer things at HomeGoods in the Spring.

Air plant stuck in one of the shells I also found at the beach.

Could be less cluttered, but this is a quick fix that I move around whenever it needs cleaning or I just want to change the look.

Assuming you never plan to use it as a fireplace, fill it with pillar candles of various heights. Tall ones to the back, etc.

Mirror the inside walls, then you can put candles, etc. in the center. Mine looks gorgeous.

I put a basket and a quilt inside mine and my cats sleep there at times.

Of course, you need to have cats......