How to get from Peace Lodge to Tambor and back to SJO?

The drive to Peace Lodge should be no problem (road is rough in spots near the Waterfall Gardens, esp. from San Miquel, but no problems if you're driving from SJO). Hope you aren't planning on doing that drive in the dark, however; that would be inadvisable. (You might consider visiting Poas N.P. volcano crater while you're up there; early in the morning, if you have time on the way back to Alajuela; 2 nights up there would be better.)
From Alajuela/SJO area, it's an easy drive to the Puntarenas Ferry.

p.s. the PL directions from SJO (hwy 1): "take the Alajuela exit. Continue straight on this
road for 12 miles following the signs for Poás Volcano. At
the dead end three-way intersection where you see Jaulares
Restaurant take a right. Proceed for one mile to the town of
Poasito and make another right at the intersection where it
says Heredia – Vara Blanca – Sarapiqui. Make a left at the
gas station at Vara Blanca and proceed for 3 miles.
Waterfall Gardens is on the left."

Note that ~15 miles will take about an hour.

Sylvia, thank you so much for your reply. No, we would not drive at night. We are taking a red-eye from Denver so we land at SJO at the absurd hour of 5:30am

You could easily do Poas early in the morning, then, before the clouds roll in! It's a very scenic drive up from the airport, you'll enjoy it (and the national park is just a few miles past Poasito).
If you do go up to the crater, and maybe about then looking for a lunch place, the Poas Lodge and Restaurant is reasonable, and has spectacular views(www.Poaslodge.com).

(And if you follow the directions from Peace Lodge for the shortcut from the car rental agencies, they are very specific, be warned!)