Humiliation ideas... You could get him to do the sissification thing in the hotel room and serve you as a maid for the evening (get room service, have him as completely dressed as he can be, you sit around and be waited on - get foot rubs, massages etc. Then, as the evening progresses, you can seduce him as if he were your serving wench.

More public idea - take him out to dinner. Have him wear women's underwear under his normal clothes and, of course, his collar. If you can get one, find a remote control bullet vibrator (one of the wireless ones, though they are expensive) and have it taped to somewhere sensitive (his shaft, over his prostate, on his balls, in his anus though the last one may be difficult to keep in place). Then throughout dinner you:

a) dress absolutely stunningly. His dream fantasy woman.
b) tease him mercilessly - play footsie under the table, eat your food seductively, make improper suggestions in whispers and so on
c) if you have it, periodically turn the vibe on and off (hopefully it will be quiet enough not to be heard by anyone, you may need to check that before you go out)
d) absolutely ban him from making any overt signs that he is aroused - no noises, sudden movements etc. Note down any he does make and punish him for them at your leisure.
e) Try to make him break the above rule. Also, try to keep him on the edge of arousal all night.
f) Tell him that whether he is allowed to cum or not that night is dependent on his behaviour.

If you do even some of the above to keep him aroused and on the edge, he should get the most amazing orgasm when you finally let him have one.