How can I repurpose half size plastic drums?

are the stong enought to sit on? You could cover and make ottoman out of and cushion on top. Or cover make pretty storage out of them. Or use as ur recycle container

Use them as planters for flowers and herbs!

I agree, with Cindy, planters would be great. You could build stands for them, myself, I would build sturdy x shaped stands with 2x4's with a 2x4 or two connecting them on the lower portion where the x starts the legs for stability. Rustic looking for barrels, but should hold them. You can finish the wood in the manner you see fit. This is my opinion, I believe the uses for the barrels are endless. If you do make it into planters, make sure you put plenty of drainage holes in them.

I would build a rack or feet for them and use as raised planter beds.

Great ideas! Can plastic be painted? They are really BLUE!

Use them for raised planters. Just make sure to drill drainage holes. Also, put a few under your gutters to catch rain water that you can use to water your outside beauties. You can also paint them to compliment your deco. Wish I had a few of them!

If they are food grade, you could use them for planters. https://www.youtube.com/watch?v=ufEQ6ZljJBE