Use the figurines of wonderous power for inspiration to give each PC a special daily power to call a conjuration that can last for the encounter and can do something
Alternatively, pick a skill or two that you (DM) is strongly associated with the spirit animal... athletics, acrobatics, endurance, perception, nature, stealth are good choices off the top of my head (or initiative if you are out of viable skill variety for the animals chosen). and for each pc, they get a bonus to that skill depending on what their spirit animal was.
As above except instead of a flat bonus, give them a once per day +1d6 to that particular skill, similar to the way a shaman does it to represent speaking with the spirits.. but don't do this if you have a shaman in the party, it would steal his/her gimmick
look at low level rituals.. and reflavor them appropriately... so if one had a bird, maybe he gets animal messenger once per day without components. if one has a horse they can cast phantom steed (or whatever it's called) once per day for free for himself only, and so on.
look to the druid utility powers for inspiration. (but don't do this if you have a druid in the party). so one with a cat might get some variation on cheetah speed (an encounter power to gain a bonus to speed for a round or something like that) or someone with a boar might get that druid utility power that increases bull rush push or charge attack, and so on.
give them all the druid wildshape multiclass feat for free (regardless of 1 multiclass limit), but can only widshape into their animal type, but still of the appropriate size requirement. then if anyone wants to bolster that further they can invest the appropriate feats to powerswap in some druid wildshape enhancing powers. this is a little iffy since someone may feel their wildshape form is less desirable than anothers (i.e. "you mean i can be a badger while everyone else is a cat/dog/horse/etc ?")
but for the simplest and easiest, i'd probably go with the suggestion to figure an appropriate skill for each animal and give the corresponding PC a bonus to said skill.