'Summer Solstice' may look idyllic . . .

Sometimes great gifts come in small packages. But sometimes those small packages contain just what they seem to contain: small gifts. On the surface,''Summer Solstice'' (ABC, Wednesday, Dec. 30, 10-11 p.m.) is a jewel, a tiny delightful surprise. It is especially interesting because it originated locally - on the Boston ABC-affiliated station, WCVB-TV - and has been picked up for national televising, something which seldom happens in broadcasting. The beautifully photographed one-hour drama wafts its way across the ABC air like a soft summer breeze, just a bit overladen with lush soft-focus , period costumes, perfect Cape Cod landscapes and seascapes, but boasting quietly triumphant portrayals by two superlative veteran actors and two excellent newcomers.

Veterans Henry Fonda and Myrna Loy sit on the beach, while new-timers Lindsay Crouse and Stephen Collins prance about as the couple earlier in life as the 50 -year marriage is reviewed in flashbacks.

What more could one ask for - fine acting, a tightly written script, intelligent direction, no attempt to stretch a slight one-hour drama to two or three hours?

Well, maybe just a little bit more. Scriptwriter Bill Phillips and director Ralph Rosenblum have managed to polish a flawed gemstone so skillfully that it may be mistaken for a perfect diamond. At one point in the reminiscing, one of the couple says: ''We're just like everybody else with one exception - we've always been able to let each other grow.''

Fine, perfectly understandable, acceptable words of affirmation and human liberation. But as the stylized, chiffony cinematography lulls you along the beach, then through the surf, floating smoothly on a kind of romantic surfboard which never overturns, the viewer begins to learn more. ''Growing'' seems to mean sexual freedom in marriage, overlooked infidelities. There is very little growth shown other than sexual philandering. Most of it is accepted with some degree of heartache, but accepted, perhaps even condoned, as a kind of miracle balm.

While it may be true that a successful marriage presupposes that the partners have learned to accept mutual human imperfections, the great revelation in ''Summer Solstice'' seems to be that the main secret of a successful 50-year marriage is integrating adultery into wedlock.

But the superbly acted little drama skips merrily along to its own melodramatic tearful conclusion. It ends where, perhaps, it might have started. Most viewers will be left with the gnawing and ambivalent feeling that what they have seen is only the partial truth, that there is much more to the story than this gorgeously pat bittersweet tale.

But is a partial truth better then no truth at all?

Seeing this unique little package is recommended here. But, believing - that's another matter.