You might be looking for a ceremony that is plain and simple, so the idea of including symbolic gestures may not be for you and that's fine. You don't need to consider any of these ideas (and humanist ceremony certainly don't need 'padding' in any way!), but if you're interested read on....
As with most aspects of the ceremony, you can include almost anything as long as it's non-religious and fits with the Humanist ethos. The most common symbolic gestures are probably hand fasting and candle lighting, But there are lost of other things you might want to consider:
There's a nice bit of symbolism in releasing balloons - about the two of you starting your journey together and about everyone's hopes and wishes for you as they each release a balloon. This looks lovely on photo of course but there are a couple of practical things you need to bear in mind! First of all, check that you have the necessary permission because of the possible hazard for low flying aircraft. And also think carefully about the ecological implications too, particularly the possible harm to wildlife. It is possible to buy biodegradable balloons made from natural latex nowadays but apparently these still take quite a long time to break down and are potentially dangerous if accidentally ingested. This is definitely something to consider carefully before you make a decision.
Planting a Tree
What more symbolism do you need in a marriage ceremony than the idea of starting new life, nurturing and growing stronger? If the two of you have an affinity with the natural world (whether you're passionate gardeners or not), this might be a nice idea to include in your ceremony, especially if it's being held in your own garden.
Releasing Sky Lanterns
The releasing of sky lanterns, as used in many Asian festivals, can represent the floating away of your troubles, to be replaced by good fortune and luck. Not to mention a beautiful way to end an evening wedding celebration. Of course, the same implications with regard to the environment apply here, but there are several types of eco friendly sky lanterns available.
Basically the world is your oyster as far as symbolic gestures go! So if you're interested in any of these ideas or you think of something original, talk to your celebrant about it. We humanists are generally an open minded lot and as long as what you're considering is ethical and non religious, you'll probably find that the celebrant is happy to include it.