There are many things they could try, to help the team now or in the future, or to clear salary space. Here are some ideas.
Rondo
to LA with O'Neal, Bass, Dooling for Gasol, D Morris, 1st
to Tor with Bass, Dooling for Bargnani, Barbosa
to NO with O'Neal for Kaman, Jack, Minnesota 1st
to Pho with Wilcox, Bass for Nash, Lopez
to Atl with Bass, Wilcox, Moore for Hinrich, Williams, Teague, Dampier
Gasol and Garnett in the paint together would be hard for teams to counter, and the Lakers get an upgrade at point guard.
Bargnani needs a new team, and with him on the floor the Celtics could surround Garnett with four 3-point shooters. Toronto might blow things up and send Calderon and other veterans away, so Rondo at point guard could help them create a new and defensive team.
New Orleans could get a new All-Star point guard, Celtics would get a huge centre they need and while they may just use him to clear cap space the draft pick could be useful for the future.
The Phoenix trade is the fun one people talk about, letting Nash have a shot at a championship as he passes to three future HoF players. Lopez is tall and can block shots. Suns get a point guard for the future to replce Nash.
The Atlanta trade hasn't been mentioned anywhere, but the Celtics would add depth and maybe clear salary while the Hawks get the passer and defender at guard to combine with Johnson's shooting.
The Celtics don't have a lot of trade chips, and Garnett and Allen's expring contracts probably help the Celtics more than the players they could get for them in a trade. Trading Rondo could bring in the size the Celtics need, and they can get by for the rest of the year with the offense being run through Pierce.