Tiny Smiles is one of my favorite cute-sy fics for Spike and Xander from Buffy. By Sabershadowkat who has many many many more fics on her website, it takes into account the time period that human Spike is from, something that very few, if any, other stories I have read do. One word for it is adorable. Below are links to each of the stories individually - all are PG-13, though some more so than others.

Tiny Smiles

Post Hush Series

From Xander's Personal Perspective

Undead Sheep

Xander watches Spike sleep. Then, later, he worries about the vampire.
(Rated PG-13)

Ooshy-Gooshy

Xander's losing his mind -- he thinks Spike is cute!
(Rated PG-13)

K-I-S-S-I-N-G

Xander. Spike. Monster. Tree. Hand-holding.
(Rated PG-13)

Demon Problems

Xander contemplates Spike's shyness, and, later, doctors the injured vampire.
(Rated PG-13)

Masimawoogle

Xander has a really good day.
(Rated PG-13)

FDA Approved

Shopping trip to Wally World. Spike's shyness revealed.
Spike's past revealed. Spike... revealed.
(Rated PG-13)

Curse of the Grape Slurpee

Grape Slushees can be hazardous to your libido. So can adorable blond vampires.
(Rated PG-13)

Eight Days A Week

A series of firsts for Xander
(Rated PG-13)

Valentine's Fright

It's Valentine's Day, and as Xander debates on getting Spike something, scariness happens.
(Rated PG-13)

Mentally Unstable Thumb-Sucking Pussycat

Xander deals with the repercussions of the robbery and assault on Spike.
(Rated PG-13)

Perchance To Dream

The gang uses the Power of the First Slayer to defeat Adam, but the spell has unforseen consequences.
(Rated PG-13)

Just An Ordinary Sunday

It's just another ordinary Sunday in Xander and Spike's lives.
(Rated PG-13)

Amy Yip, at the Waterslide Park

The boys are off to the waterslide park.
(Rated PG-13 barely)

The Chip

There is a possibility that Spike's chip will come out.
(Rated PG-13 barely)

A Choice Made

Spike makes a choice that changes Xander and his relationship.
(Rated PG-13 barely)

Cherry Boy

Xander makes a discovery.
(Rated PG-13 barely)

The Love Boat: Welcome Aboard

The boys go on vacation.
(Rated PG-13 barely)

The Love Boat: Formal Dinner

The boys continue on vacation.
(Rated PG-13 barely)

The Love Boat: Cliched Romance

The boys get mushy on vacation.
(Rated PG-13 barely)