Some ideas:
1) Leave a cigarette butt in a urinal or floating in a toilet at the Kingdom Hall during any of the non-public meetings. Repeat on a weekly basis. Losing lottery tickets could also be placed in semi-inconspicuous places
2) Hack into the sound system; place a hidden radio receiver that can be switched on by remote signal. Transmit various enlightening information. The possibilities are endless.
3) If the library has audio or video cassettes, these can have segments re-recorded. Fun, fun, fun. Extra credit if an "improved" cassette recording of a Kingdom Melody is played during a meeting.
4) Place a copy of _Crisis of Conscience_ in the library, but first have it re-bound with a cover from some WTBTS book. See how long it lasts. Repeat with bindings from other WTBTS books.
5) Religious tracts from other groups can fit nicely into the contribution boxes.
6) A contribution box might also be improved if one were to deposit a floppy disk with a bunch of HTML pages from this and related sites.