Tiny outdoor space ideas!

Water is also a must, IMO. We have a feature that works by solar gain and recycles the water, so no electricity or plumbing needed.
If possible, a little pond could be made from an old bath or tub. Then apart from attracting wild life, you could plant some lovely water plants.

Which direction does it face? Does it have high walls/fences/hedge, is it overlooked?

scented - yes! I hadn't thought of that yet. Definitely need some folding chairs

It's pretty much square - around 25-30 sq metres. Pinterest is my new best friend. The sad plant shelf sounds like a great idea!

We have a tiny garden and the first thing I would do is get rid of any lawn if there is one.

We use lots of evergreen plants/shrubs for structure (as J52 suggests) and interesting climbers. Then, pots are your friend - filled with colour from bedding plants adding shrubs and perennials as and when you see/acquire them. Scented plants work well in a small garden especially if it is enclosed.

You will need a bench or some sort of seating too - that's what gardens are for!!

What a great project, but You've not given much info about the aspect or structure of the garden, but as far as plants go......
It’s worth thinking about putting in some perennial structural plants that will last a long time and be value for money, in the long term.
I’d also look at the sad plant shelves of the garden centres or Homebase. Often these reduced plants perk up considerably when looked after.
Also, ask friend for cuttings of plants that do well.
Have a look at small gardens on Pinterest.