Theme: A Trip Down Lover's Lane - Follow the three steps found at the end of the first theme entries and you will arrive at the fourth.
17A. Soft hit that barely makes it over the infield : BLOOP SINGLE
28A. Archaeological age-determination process : CARBON DATING
45A. Completely absorbed : FULLY ENGAGED
59A. Sign in a limo that aptly concludes the sequence formed by the last words of 17-, 28- and 45-Across : JUST MARRIED
Argyle here. Straightforward Monday. A little something for everybody. Easy to find cheater squares. An obscure name or two. A bit of crosswordese. If you can't find a nit, you're not trying but you would be hard-pressed to say you couldn't get it done.
Across:
1. Apt. parts, in ads : BR's. (bedrooms)
4. Talking head : PUNDIT. Originally meant "learned Hindu" before sinking down to a talking head.
10. Big name in ATMs : NCR. (National Cash Register)
13. Charged particles : IONs
15. Black-and-blue mark, e.g. : BRUISE
16. Suffix for pay : OLA. Payola/Graft
19. Cranberry-growing area : BOG
20. Africa's Sierra __ : LEONE. On the west coast. Map.
21. Fed. retirement org. : SSA. (Social Security Administration)
22. "T" on a test, usually : TRUE
23. Like dodos and dinosaurs : EXTINCT. But not 'our' Dodo.
26. Foray : INROAD. (encroachment, raid)
31. Texting units: Abbr. : MSG's. (messages)
34. Rowboat mover : OAR
35. Wish granter : GENIE
36. "How was __ know?" : I TO
37. Abrasions : SCRAPES
40. Sinus doc : ENT. (ear, nose, and throat)
41. Not exactly robust : FRAIL
43. Simpsons neighbor Flanders : NED
44. Makes really angry : IREs
49. Lawyer's customer : CLIENT
50. Accessory often carried with a wallet : KEY CASE. So you won't jangle when you walk.
54. Merle Haggard's "__ From Muskogee" : OKIE
55. N.J. neighbor : DEL.
57. Lightened : EASED
58. Libertarian politician Paul : RON. And small-government proponent. (yesterday)
62. Mystery novelist Grafton : SUE. 'S' is for Sue.
63. Houston team : ASTROS
64. Statistician's input : DATA
65. NHL tiebreakers : OT's
66. Tinkers (with) : MESSES
67. Figs. : NO's. (figures/numbers)
Down:
1. The Good Book : BIBLE
2. Pricey watch with a gold crown logo : ROLEX
3. Nose-in-the-air type : SNOOT
4. "Nova" airer : PBS
5. Ocean State sch. : URI. (University of Rhode Island)
6. Convent dwellers : NUNS
7. Starts to eat with gusto : DIGS IN
8. Manhattan is one : ISLAND. The Manhattan I like.
9. Golf ball's perch : TEE
10. Choice you don't have to think about : NO BRAINER
11. Metaphorical state of elation : CLOUD NINE. This site explains (sort of) where the term comes from.
12. Violent anger : RAGE
14. Former (and likely future) Seattle NBA team : SONICS
18. '90s Cabinet member Federico : PENA
22. Lug : TOTE
24. Gator's kin : CROC
25. Skier's way up : T-BAR. Back to our usual.
27. Glad __: party clothes : RAGS
29. Long-armed primate : ORANG. Informally; an orangutan.
30. Comprehends : GETS
31. Tick off : MIFF
32. Went down swinging : STRUCK OUT
33. Touchdowns require crossing them : GOAL LINES
37. Leonard __: Roy Rogers's birth name : SLYE. Trivia.
38. Mountain top : PEAK
39. Advantage : EDGE
42. Nastase of tennis : ILIE
44. Security checkpoint request : ID CARD
46. Ultimate application : END USE
47. Big bomb trials : N-TESTS
48. Binoculars user : EYER
51. Made in China, say : ASIAN
52. Look after : SEE TO
53. Icelandic sagas : EDDAs
54. Estimator's words : OR SO
56. P.O. box inserts : LTR's. (letters)
59. Printer problem : JAM
60. Stooge with bangs : MOE
61. Pack animal : ASS
Argyle