So, This is really not like myself (not that mainstream) but I would like your thoughts . For example for a day at the beach or something. Your 5 songs must know, to make a "jolly" afternoon/end of day.
If you mean acoustic instrumentals that perhaps impress people I would say

1. Classical Gas (Tommy Emmanuel arrangement) as an energetic and happy piece.

2. Level 5 by Ewan Dobson as a jawdropping and awesome tune that would make people admire you.

3. Minuet in Fingerstyle arranged by Alex Kabasser since everyone recognizes it and the melody is awesome.

4. Tetris or Super Mario classical or fingerstyle version just because they're so awesome and everyone must have heard them any some point in their life.

5. Any tune where you tap on the neck with both hands and make slap harmonics just because it looks cool and hard. Like some Antoine Dufour or Andy McKee tunes since they are pretty reconized.

But seriously, just like Sweet Home Alabama, Freebird, Wonderwall, Snow and To Be With You are perhaps nice.
Rather a lot depends on the type of music you're into. I could try to trot out all those tunes at my local folk-jam and the guys would look at me funny.
They'd expect traditional stuff....
Last Kiss - Pearl Jam

Here Without You - Three Doors Down

Fast Car - Tracy Chapman

Second Chance - Shinedown

In The City - Tom Petty

I think you all forgot "Wonderwall". Everyone knows that
Don't forget "Don't Stop Believing" by Journey - Fingerstyle of course. Find me someone who can't sing the chorus and i'll give you $100 :P
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Quote by JKHC

I think you all forgot "Wonderwall". Everyone knows that

Someone posted it I know wonderwall anyways and don't look back in anger