I have been looking into this as well - we have a couple of days to kill at the end of January also! I guess it depends on what your idea of "too expensive" is... Here is what I have found in looking for our family of 5...
If purchased in advance online, Universal offers a 7 day unlimited ticket to Universal Studios, Universal's Islands of Adventure & Citywalk for 94.99 - if you use it all 3 of your days, that's less than 20.00/person. The last time we went to Universal, we spent a full day in each park and another half day prior to our flight. Never made it to Citywalk.
Seaworld offers a Funcard for 74.99 adults, 64.99 for kids which gives unlimited admission for all of 2009.
There is also a Seaworld/Busch Gardens combo for 100.00 good for unlimited admission for 2 weeks from first use.
Wet n Wild offers for 41.95 unlimited admission for 2 weeks from first use. Weather in January can be iffy though...
We enjoyed Silver Springs - it was a nice change of pace from the big theme parks, but your 13yo may be a little bored there. I believe they have one free child ticket with each paid adult ticket (29.00).
My kids also liked spending a few hours at Gatorland - their pricing is around 18.00 and 2nd day is free.
Downtown Disney has Disney Quest - depending on your what type of WDW tickets you have, they may include this.
I could go on and on - lol...
Check out the Orlando Visitors Bureau web site - they have discounted tickets to many attractions in the area and will give you an idea of the lesser known attractions - there are just way too many choices!! They also offer a Go Orlando card which gives admission to a variety of local attractions. I still don't know what we are going to do! lol
Hope this helps at least a little!