Though CDR doesn't fit the veteran mold, I would give him an invite to camp.
Douglas-Roberts struggled with injuries last year, only playing 49 games excluding a few playoff games. But he seems like another solid perimeter defender and is an average three point shooter to boot. Allen or CDR would be a nice depth signing, when you consider Lebron is getting older and older as time passes, adding depth at the wing may be wise to preserve his health.
Gustavo Ayon is another interesting name in FA.
Though he struggled with a shoulder injury last year, Ayon's passing and rebounding would fit great with the many good passers and rebounding bigs on the Cavs, redundancy with solid passing big men is never a bad thing, ask the Spurs. This team is shaping to be one that should lead the league in multiple statistical rebounding categories, even without the signing of Ayon. Although, he wouldn't solve the Cavs' defensive or rim protection problems, keeping Varejao fresh for the playoffs should be as much of a priority as any. Gustavo is capable of providing 15 minutes of decent center play when healthy, such as he did with Atlanta.
Finally, Ray Allen, he averaged 26 minutes per game being Miami's key contributor off the bench for a team that won a title one year and made it to the finals the next. Allen turned 39 in July. Let that sink in.
No one can predict father time, but even he can't take away the mentoring Allen could provide the young guards on the Cavs. Also, he's still shooting threes at a high clip, 41% his first year in Miami and 37% the second. His career average is 39%, which he isn't far off from. His defense is suspect, but as a reserve SG playing along Delly, Marion, Waiters, James, the list goes on. How is he not playing defense going to have adverse effects on the team?
Especially if the Cavs acquire a rim protector.
So to me the positive way outweighs the negative with him. More bench depth is never a bad thing, let alone if an all-time great three point shooter is checking in the game for the Cavs.
Doesn't look too old to me.