I am not big on spending much on party favors either. The way I look at it, the child was invited and the party is their treat. I know, many moms would gasp at this. I woudln't feel bad if you just weren't in to it. At her 2nd birthday party, I just gave a little bag with some dora fruit snacks and a little ball (we didn't really have a specific theme).
However, in thinking about my daughter's next birthday party (she wants a "doggie" birthday party), I kind of think I might enjoy putting a little thing together just because of the theme...makes me a little more excited to put something together. Like putting those Scoobie snacks (dog bone shaped cookies) in there, etc. My friend had a wide age range like you, but she still followed the theme. It was a pool party with a mermaid theme. For the older kids she got big cheap water guns as their favor to play with at the party.
Do you have a theme? Maybe start there for some ideas.
Also, maybe you should just focus the goodie bags/party favor towards the little kids. My brother is 16 now and I never thought about giving him a goodie bag at my daughter's birthday party. I think the "big kids" are a little old for that and proably won't care too much about not getting a little bag of toys and candy. :)