I already mentioned how I use resistance bands for strength training, using them primarily for upper body exercise, while also doing push-ups, and bicycle crunches.
Here are some other things I use for strength(or resistance) training:

Use a walker to do dips: Just like the dip bars at the gym, you can use a walker to do the same thing. Great for improving upper body strength.

Juggle with heavy balls: These balls are tennis balls that were stuffed with pennies and sealed shut with epoxy and duct tape. Not only do they help strengthen the muscles used for juggling, their weight may also make a deeper imprint on muscle memory so that I juggle better. They weigh 1 pound each, which isn’t much but after several minutes it does get tiring.

Ankle weights: A necessity for building strength in the legs for runners. But they shouldn’t be used while running, that can increase the risk of injury. This is the All-Pro brand, I’ve had them for almost 10 years and they have served me well. The velcro has deteriorated a bit unfortunately, but it’s not a major problem.