1.  Invisible ink message in a bottle
2.  Crystal hearts
3.  Fizzy hearts  
4.  Leather keychain
5.  Painted mason jars 
6.  DIY envelope clutch
7.  Homemade lotion
8.  Homemade soaps
9.  DIY canvas tote 
10.  Framed Valentine’s gift
11.  Photo transfer art 
12.  Memory board  
13.  Valentine’s Day Cube
14.  Paper roses 
15.  S’mores Valentine’s Day jar
16.  Citrus lip balm
17.  Tea blends
18.  Coffee cozy
19.  DIY hand warmers
20.  Homemade scrapbook

Cites:

https://www.hellowonderful.co/post/10-brilliant-valentine-science-experiments/

https://www.bhg.com/holidays/valentines-day/cards/make-your-own-valentines-day-gifts/