Jon-At yur daughters' ages, the simpler the event, the happier they will be. Anything that requires a long ride anywhere starts kids off bored, so the end result must be hands on for them, like a children's museum where they can try out different things and touch everything.
Our local fire department had an exhibition where the life flight helicopters were available and we got to ride in a fire truck...my son still remembers this 12 years later, plus it was free. Airports sometimes have planes available for kids to check out. Anything involving water is fun. Do you have a YMCA in your area? It is cheap to pay for a swim with the kids one night a week. Just walking through the neighborhood at the kids pace would be fun, you could observe and comment on 'look at this daffodil, it is really ruffled like a lion's mane, but this one is hardly ruffled at all'. We dragged a magnifer out once and squated by the side of the road looking at different kinds of beetles.
In my small town, the newspaper still reigns as king versus online media, maybe yours does too. The paper often lists what is going on locally in the events section of the paper. A lot of those things are free.
Try also meetup.com to see if anyone in your area has started a meet up for parents with small kids, or singles with small kids, they may have structured events you can do...or you can start your own meet up.