Christmas tree light switches are yet another way toddlers can get safely (well, relatively safely) involved with the tree. Your mileage may vary given the activity level of your toddler, but both Christy and Carly have had good luck:
I found these Ornament Christmas Light Switches a few years back and notice that Target carries them for $9.99. [More expensive at Amazon, but I've linked to it in case you must have one and can't find it near you. Great item to buy locally on December 26! — Ed.] I think I bought mine at a drugstore for $5 after Christmas one year. Unfortunately the brass tarnishes after a few years, but that's been the only downside.
These work great for us because the kids can turn the tree on and off at a whim, and for added fun, the baby just hangs onto the ornament while we touch her and it turns the tree on and off. This keeps everyone entertained for hours on end.
We picked up a $4 foot pedal button extension cord that turns the tree lights on and off — which our daughter adores doing. Its lengthy cord means a) our toddler is nowhere near an electrical outlet, and b) is very easy to use with a big red button (parents with younger babies may find this a detriment, however!)