Second the Canyon Bistro -- non-buffet Sunday brunch stuff (salads, soups, sandwiches, egg dishes, etc.) from 10 - 5. In the same complex -- the Pine Tree Circle, at the intersection of Topanga Cyn. Blvd. and Old Topanga Cyn. Rd. -- is the Water Lily Cafe (coffee, sandwiches, etc.), from 9 - 5 on Sundays. Inn of the Seventh Ray is good, but not what I'd call light (or inexpensive); Abuelita's is also good, with a very nice patio, but likewise not particularly light.
You might also think about dropping into the Topanga Creek General Store (aka Joe's Market, even after all the years since Joe sold it) before your hike. Back at the meat counter there is a nice selection of prepared sandwiches and other things (deviled eggs, salads), made there in the store. Pick up some of that stuff and some drinks, and have a picnic while you're hiking.
By the way: you don't say where you and the pals will be hiking, but let me suggest the Red Rock area. Take Old Topanga Cyn. Rd. to Red Rock Rd., turn the only way you can (west, more or less) and follow the road all the way to the end, where there is parking. Walk up the fire road, which if you follow it all the way will take you to Stunt Rd., or turn off onto one of the trails that spur off up into the hills. Too late for many wildflowers now (you should have seen it in the spring!), but great hiking nevertheless.
there's a great breakfast/lunch place called pat's - 1861 N. Topanga Cyn. Blvd. nice setting, great service (although slow at times - this is topanga), reasonable prices and pretty tasty.
froggy's is also pretty good - but i like the inside of that restaurant better than the outside. the crab enchiladas at abuelitas are v. tasty - but not light.
do you want to eat in the canyon, or are you heading back to the valley or coast after... would you drive out to eat somewhere?
agree inn of the seventh ray meets the light criteria, but it is a bit pricey, and you may not want to head there after a hike...
in topanga, there's froggy's fish market, which is good with light options. it's also next to a juice bar that offers vegan meals, but the name escapes me right now. there's also abuelitas, but not everything is light... ceviche and salads, as well as fajitas though...
you could head into malibu and hit up Malibu Seafood for some light casual seafood. there's also Cholada Thai on pch west of topanga
in the valley on topanga is Follow Your Heart
I'm sure you are going to get some recommendations for the lovely serene setting of the Inn of the Seventh Ray. Yes, it can be nice and romantic with sage burning and the brook babbling and all of that. My problem with it is that it is pricey, I've never had food there that I really enjoyed, and it can be a bit difficult to find.
Close to the foot of Topanga Canyon as it meets PCH is the Reel Inn. Casual, order at the counter and find a table kind of place. As long as you keep your fish simple and order it grilled, it can be satisfying. Most items range from the low teens to about $20.