Beltane is opposite and complimentary to Samhain (halloween), so it’s sometimes considered one of the two main celebrations in the Wheel of the Year.
Just like Samhain is sober and solemn, Beltane is joyous, playful, and open-ended. Anything that sparks your creativity goes! 
Traditional correspondences have to do with fire and sun. Beltane comes from the root bhel, which means “to shine, flash, or burn”. You could make the god Belenus the centerpiece of your altar, he was a Sun god 
known as “the bright one” by the Celts.
Here’s an altar for the Sun I improvised last year:
Here’s a Forum topic on How to Celebrate BELTANE Alone? Ideas! 🔥
The maypole and colorful ribbons are another key theme of the season. Explore Pinterest for more inspiration! Here’s a link to Beltane Altar Ideas on Pinterest