There are several interesting designs for low-cost outdoor balance beams.
Using pressure-treated 4x4s and patio footing hardware, you can install a simple balance beam. You can interconnect any number of 4x4s, as shown below.
The patio hardware can be concreted in.
There are a variety of permutations possible off this simple design.
I observed another interesting balance apparatus in a playground in northern Uganda (installed by East Africa Playgrounds, an English NGO). It involves the creative use of old tires.
A possible permutation of this design involves a tow strap stretched between vertical short sections of log. This design could also be used in a slack line configuration, with a thinner strap replacing the tow line.