In our house we had to basically STOP all activity (except for special occasions) after 6PM. If it were my house, I'd move the walk to before suppertime and have after-supper be nothing but quiet and slow. No TV or videos after supper either in our house at that age; only recently been letting the girls have one pre-bedtime show. (Mine are now 5 and 8.) Maybe some soft music in the background if it helps keep the mood calm, or some music at bedtime. Both my girls have bedtime CD's they've helped DH select music for from iTunes and burn onto a disc, but the first few discs DH and I did for them since they were so young yet.
Oh, and my children have taught me that reward charts aren't all they're cracked up to be. Life got so much simpler when we ditched rewards and punishments altogether and went to us just telling them what we needed and why, giving them input - but that's another thread on the Companion Books and Methods forum (the Unconditional Parenting thread), and I'm not trying to make this go that direction. It's just that at 3YO, I have to agree with PP that she isn't really "getting" the system.
Have you talked with her in an age-appropriate way about why sleep is so important for her body to be healthy? Maybe find some books at the library that talk about sleep and bedtime to reinforce the point. Both my girls also get massages at bedtime, with joint compressions when they're really wound up; squeezing the joints seems to help them get more "grounded." Not big long elaborate ones, maybe 5 minutes total, but it makes a big difference. Combine that with soothing music and they're usually out, or at least settling down, in 5-10 minutes.
Is she perhaps staying up in an effort to get more Mommy time? How much exclusive Mommy time does she get during the day, especially after supper? Don't know if that's the issue, just kind of shots in the dark.
When Josie and Nat had issues staying in their rooms, we went thru a period of time where we kept a sleeping bag on the floor at the foot of our bed; the deal was that they could come sleep there if they woke during the night as long as DH and I could keep sleeping, with exceptions being something hurting or bad dreams. Plain old "I want to sleep with Mom and Dad" wakings went straight to the sleeping bag. Ah, bliss - SLEEP for us!
I'd also shoot for an earlier bedtime, like more in the 7:30 neighborhood if possible. How bright is it outside where you are at that time? (I'm northern hemisphere so full-on long-day summer now, but southern not so much.
Can you keep her room fairly dark?
Can your DH put her to bed, or at least get her bedtime started, so she and the baby can go to bed at the same time? DH and I have tried to alternate putting our kids to bed, taking turns with them and switching off every night or two, so if one of us needs to be out, the girls are comfortable with either of us.
As a last resort, if you're able to get melatonin where you are (we're in the States and can get it at Whole Foods in a nice peppermint-flavored sublingual), a small dose of that might help you break the cycle. The ones we have, the adult dose is no more than two tablets, and when they're keyed up (or if we know they need to go to bed earlier for an earlier morning wakeup, for example), 1/4 of one tablet is more than enough to help them settle. We don't use it long-term, but it did get us out of the wild-bedtime routine last summer; this summer is so much more pleasant.
