For a quick up-and-down, Avoriaz would be good. Basically up the road to Avoriaz - it's about an 800m climb - then either back down the same way, or hang a right and drop down the Joux Verte via Les Lindarets and Montriod.
A good half-day loop would be Morzine - Col d'Encrinaz (road round the back of Mont Chery to Pont du Gets then the back road down to Taninges. This keeps you off the D902 via Les Gets but is a bit hilly. You then head up the Giffre Valley to Samoens and up over the Col de la Joux Plan, which drops you back into Morzine to finish. Cols don't come much harder than the Joux Plan; it's steep, unrelenting, hot (south-facing) and has a pretty uneven gradient, and a long open pull at the top when you can see the restaurant at the top from a long way away. The descent into Morzine needs care; it's steep, narrow with poor sight lines.
The Ramaz is probably the nicest col in that area; the ascent from Mieussy via Sommand is ace - starting from Morzine you could hit it from the Vallee Verte (loop round from above St-Jean d'Aups to Megevette then head towards Mieussy; you can pick up the main climb above Mieussy and then come back over the Encrinaz. I think that's the longest of the loops. Ramaz from the Pont du Gets side (via Pras de Lys) would be horrible I reckon; it's steep and in the trees for a lot of it.
Hope that helps.
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