“Since this is the time to ask, …
Is there anything that I can do to protect any assets, such as 401k, collectibles, etc, before I get married, assuming that the spouse will not sign a prenuptial agreement?”
To the extent possible, keep all titles in your name alone. Make an inventory of all the things you desire to protect, and keep a hard copy of that inventory in a few safe places (maybe with pictures). You could set up a trust and put the protected things in that trust. If possible, have the Plan Administrator of your 401K start a separate account for you for contributions made after the date of marriage, so you can easily segregate out contributions attributable to marital efforts.
Your soon-to-be-spouse doesn’t trust you enough to sign a pre-nup even though you really want one? Are we starting out on the right foot? Is communication an issue? Though this may be the “time to ask” your spouse and/or your counselor may be the ones to ask.