Serpentine belt can, in general (not sure about this car in particular) be done in a driveway by any idiot who knows which end of a wrench to hold. Which also means it can be done by a mechanic for probably +/- $100. Or free if you get the timing belt done (ie, I'd only pay parts for the belt, not labor, and a belt should be +/- $20.)
Timing belt is a much more involved process. On the Sienna, the engine is an "interference" design, meaning if the belt breaks, the valves hit the pistons and you buy a new engine. Personally, that's not something I'd mess around with, I'd either get it done/do it myself, or sell the van. The interval for a change is 90k miles, so you're either 18k over or 200k over (or, I suppose, 108k over?) A timing belt break would be catastrophic, engine seize, and you stop right there, no more moving. I wouldn't drive a car on the highway much more if I'm 18k or more over the interval on that one.
So really, up to you to pay or not, but I would suggest your options are fix or sell; I would not suggest you continue on driving it anywhere it would cause an issue (highway) if it was suddenly dead.