Got a budget? What do you consider "inexpensive"?

Do you have kids? Or is this space mainly going to be used by adults?

I have young adult kids but a very large family who enjoys the outdoors. If I can get away with doing something for under 5k that would be ideal but highly unlikely

"something for under 5k that would be ideal but highly unlikely"

Depending on where you are located, that would be very unlikely for just one of your desired options, let alone both of them. You could probably eek out a firepit area for 5k but it would be pretty rudimentary and not very polished looking.

You can also get a very simple recirculating water feature for that budget but the size of one for that price would need siting pretty close to the deck to be appreciated at all. And that's assuming the electricity is available. If not, that will add to the cost.

You might need to rethink and see how far you can expand that budget :-) And maybe focus in on the one feature you think will be of most use/value to you and the family (firepit area?) and do it up properly.

Pick a range of flowering natiive understory trees and shrubs and add some color. Clethra, Calycaxnthus, Mt. Laurel, etc.

A fire pit generally needs a patio.

Croquet, Badminton, Volleyball. A vegetable garden. Show the kids how to grow some side dishes and learn where food comes from. An herb garden for the chef. A quiet shade garden and reading room/deck/bench in the corner.