As a former preschool teacher I'd like first to point out that toddlers are known for having little to no attention span ADHD or not.
As a teacher that spent time in a specialized classroom with middle school students where a majority of my students had severe ADHD in addition to other learning/behavioral disabilities, the first and most important things are those you mention in your question, and just spending quality and positive time together which builds his confidence and self-esteem.
In addition, regular meditation can help improve focus but I have no idea how to get a toddler to meditate. You can also check out children's yoga programs as the physical activity combined with stretching can be a good decompressor between activities that require more focus and/or sitting relatively still for longer periods.
Also, more and more research is showing that ADHD isn't really a lack of attention as much as it is attention on too many things at once. They can often actually look like they are not paying attention at all and still do BETTER on information retention after a learning activity than their non-ADHD peers if they are using a few techniques that allow for smaller movements that are not distracting to others around them. This article is really for school aged children, but might be useful to you anyway.
Activities that reduce stress can also help relax children so they are more likely to focus. Although these are not activities that help "fix" or "improve" attention spans or focus long term, sensory activities can be soothing and help them to get a breather from distractions to prepare them for a period when they will need to focus a little more.
Sensory Activities also help with a number of other skills like stress management and muscular control. These are probably the closest activities to what you are looking for.
It can also help them all through their preschool and school years to have a "fidget" to use to focus their energy into a small, non-distracting fidget they can use while listening to something else.
Also, aside from the things you mention as "general" I will give you a couple of additional environmental factors that can help reduce stimulous factors that can be disruptive to attention span that you didn't mention. Earthy but light pinks, greens and blues as the main colors on walls are helpful to increase attention spans for those people in the environment. There are special lightbulbs that can also help with this (just the right range of wavelengths and no buzzing - I'm really sorry I forget what they are, but you could probably ask on the Bridges Academy (California) page because they use them) and minimize clutter. Make sure your kid's bedrooms have only a few pieces of artwork and at least one wall that is not "decorated" by anything.
I hope you'll find this helpful.