Planning a fun 40th birthday party can be easy and memorable. Consider a theme that incorporates the decade the guest is turning like the 1980s or 1990s. Activities don't need to be extravagant - simple games, great food and music from their young adult years will have guests feeling nostalgic and celebrating a major milestone. Decorations and photo props capturing the era when the guest was in their 20s will make for lots of laughs and memories to share on social media.